The purpose of this post is to describe how I set up my gear and why. Everybody's needs are different, so it's not to say that this is the best way to go, just what works for me.
I just took a class today using a VTAC Brokos Belt with VTAC Cobra belt for the inner belt, with 3 HSGI taco mag pouches on my left side and three handgun tacos attached to them, and an Atomic Dog holster on the right side, and a dump pouch in the back that I use to hold my gloves and empty mags. I think this setup is ideal for someone who doesn't need more than 4 rifle mags, and who doesn't wear armor. If extra mags are needed, they go in the left cargo pocket. It's relatively comfortable even with all the weight on the hips since the belt spreads the weight over a large area. And you can put on and take off the whole rig in about 1 second using the Cobra buckle. I don't have to change anything to switch to a different rifle system since the tacos can hold any rifle mag. This is very nice for me because I usually run an AK, but sometimes I run an AR. My dump pouch is a cheap 5.11 one. As far as I can see, there is no need for anything fancy in a dump pouch. The gloves I run are Mechanix MPact, which have been working well for me. I would recommend at least having gloves on you if you're going to run a carbine, since the handguard and barrel can get very hot after rapid fire. A quad rail AR handguard can be sharp, and an AK especially has lots of sharp edges all over it.
The HSGI tacos are expensive but really nice. They fit any rifle mags, the retention is adjustable, you can attach a pistol mag pouch to it, you can run bullets forward or backwards as you wish without changing anything, and they're fast since there's no retention device like a flap or bungee strap.
The Brokos belt is a lightly padded MOLLE belt that uses an inner belt to provide the tension around your waist and a buckle. It buckles around your waist over the top of your pants belt(which is not the same as the inner belt). The Brokos belt has the nice feature that you can expose the inner belt to the outside and use your existing OWB holster or other belt mounted accessories. It's also light and comfortable. The downside is that it isn't really anchored to your pants belt, so it can ride up a little bit. I found that it only rides up on my hips when I drop to prone, but then when I stood up, it automatically went back down to its normal position without me having to adjust it in any way. So it wasn't really a big deal.
On sizing: the inner belt should be a little bit longer than usual, since it has to go outside your pants waistband and your pants belt. The Brokos belt can be quite a big longer than usual, since that just means that the MOLLE extends closer to your centerline.
I wear 34 or 35 inch waistband pants, and I have the Brokos belt in XL, but the inner Cobra belt in Large. I originally tried the XL Cobra belt but it was too long and the excess was getting in the way of my holster. The Large Cobra belt is almost a little too small, but is workable. If I were to do it again I would probably go with the XL and just trim it down.
I don't like chest rigs because they can be uncomfortable when you go prone. Also, if you're training in the rain or cold, if you have a chest rig, you have to take it off and on when you want to add or remove layers of clothing. You may or may not need to do the same thing for the battle belt, but a battle belt comes off and on much easier.
I would avoid any double(meaning two mags in one compartment, like this: http://www.cheaperthandirt.com/MOLLE940-1.html) mag pouches because if you take one of the mags out, then the other one is loose and can fall out, unless you remember to reapply the velcro or other retention device. Each mag should go in its own compartment that has its own retention.
Kydex mag pouches are great but if you have too many of them with fully loaded mags on your belt, the weight becomes uncomfortable and most designs don't have any way to attach pistol mag pouches.
Some people would say that you should 'train as you would fight'. I would agree with that, except for the fact that in my lifestyle, I wouldn't really ever use the carbine in a fight unless it was a serious SHTF where anything goes and more gear would be appropriate. I leave my carbines in the gun safe and I have a quick access lock box with a handgun in it for emergencies. That said, I do think that beginners to carbine training should keep their gear simple and inexpensive so they can look at what other people are running and get an idea of how it works.
For a relatively slick setup, I'd probably just go with one or two kydex Ready Tactical mag pouches and then run the rest out of pockets, and have one dump pouch for empties. The Ready Tactical pouches are relatively inexpensive and you can run your AR mags bullets pointing forward or to the rear. I prefer pointing to the rear, but that's something each individual has to decide for themselves. The LAPolicegear.com 'operator pants' have two pockets that are just the right size for AR mags and will fit AK mags too. They're the horizontal slits right below the plastic D-rings on the belt loops. http://www.lapolicegear.com/elastic-...ator-pant.html
For info on carbine legality in California, see http://www.tenpercentfirearms.com/index.php?main_page=ar15
Between featureless carbines and bullet button carbines, I would definitely go for featureless. Being able to use the mag release normally is well worth not having a flash hider or collapsible stock or the other evil features. On the negative side, I have found that not being able to wrap my thumb around the grip makes it harder to do rapid fire consistently. Also, on an AR, you're going to want to install an ambi safety if you're right handed, since your thumb will be on the right side of the rifle. Featureless rifles can also be used with legally owned pre-ban magazines, while bullet button rifles can not. I like the Solar tactical grip wraps, which bolt on to a normal pistol grip and therefore can be easily removed if you visit or move to a free state.
If you go the bullet button route, the gpik works reasonably well:
You stick it on your middle finger and can use it to drop the mag. It does require you to be fairly precise to poke the bullet button in exactly the right place. One caveat: if you have to draw your pistol, it really messes with your grip.
If you are running 10 round magazines, I would recommend getting 10/30 magazines, which are 30 round magazines that have been internally blocked to only hold 10 rounds and then permanently fixed so they can't be disassembled. The reason I recommend 10/30 mags this is that they will fit in normal pouches, and you can get a much better grip on them for malfunction clearing and tactical reloads, compared to short 10 round magazines. Solar Tactical has 10/30 mags or the kits to make them. If you want to make them yourself, you'll need disassembled mag rebuild kits, which can be had from Solar Tactical, or http://www.copesdistributing.net, http://www.calegalmags.com/, or http://midwestpx.com/, among others.