Well, there are a number of barrel profiles.
1] Standard profile.
(aka "pencil", "superlight")
This is a .625 diameter barrel, it was what the M16 originally had. It is light, does the job. Today, you can get this type of barrel from either Colt or Bushmaster (actually a little thinner), and a couple of other places in 16" length.
2] M16A2
(aka "government")
This is the 20" barrel profile of the M16A2. It is rare as the commerical market doesn't make many of these. These are thin under the handguards, then .750 from the FSB on out.
3] M4
These are 16" barrels (assuming commerically made), thin under the handguards, then .750 outside the handguards. Has a cut out for grenade launcher attachment.
4] HBARs
These come in many different lengths. They are all .750 from the front sight base to muzzle, but they vary in thickness under the handguards. 99.9% of them are much thicker than .750 under the handguards, which makes the barrel heavy and the rifle muzzle heavy. These are purely a commerical concoction that exist solely to save manufacturers the cost of having to turn down a barrel to a smaller diameter. Less lathe time, less tooling, less waste. They are so popular, that most people have considered these to be the "norm"..and thus why other barrels listed above have nicknames like "pencil" or "superlight".
5] Bull barrels.
These are usually your 1" thick diameter barrels that people use on precision rifles. They want the thickness for stiffness. Often, these are scoped rifles with bipods meant for bench shooting or precision work.
There are a few commerical hybrids, and a couple military variations I didn't mention, but those are the main ones.
My recommendation. Go with the standard "pencil" barrel. There's absolutely no need whatsoever for the extra weight of a stupid HBAR barrel. You will hear every single justification for HBAR's, like:
1] They are more accurate. - this is false.
2] They resist heat better - this is false.
3] They balance better - this is opinion, and I'd say a really poor one.
4] HBAR's are ok, after all, you're a wimp if you complain about their weight.
5] Be a man, and carry the extra 1lb.
6] Makes the AR "feel like a real rifle"
Hbars are not more accurate, they take longer to heat, but they also take longer to cool - their resistance is the same, about the only thing they are good for is absorbing more recoil, but on a .223....yeah, exactly.
Most people need to justify the HBARs because they already bought one. I owned one and sold it. Once you go standard, you won't go back. Save yourself the trial and error, and go with the winner from the start. The standard barrel will do absolutely anything the Hbar will do, and do it with less weight. There's a reason Stoner invented the rifle with a .625 barrel.....
HTH
and I have read that the chromemoly barrels, if taken care of, will work just as well as the chrome lined. I recently bought my first AR, and I love it... it's a great little weapon. 
