I'm new to this forum and and old guy. I had a M-16 in Vietnam and decided I want to get a Colt AR15. When I look at their site, there doesn't seem to be that much difference between the models other than cosmetics. I've got a knowledgeable friend who said as long as I got a Colt, the models were pretty much the same. I know I'm showing my ignorance, but which should I get and why?

FWIW I'd go with a Bushmaster, LMT or Rock River Arms rifle and save some money.

Colt has all manner of quirks in varying models (small pins/large pins, receiver blocks etc...) while Bushmaster rifles and LMT rifles are considered mil-spec.

Mike

After you read this site for a while, decide on what type of shooting you want to do, what barrel length & profile you'd like, what ammo you intend to run, custom upper or stock handguards, and 4500 other questions, go to the Equipment Exchange and order it.

For example, here is a rifle I call my 100% AR15.com EE rifle, got the LMT lower from Pete at Legal Transfers, the upper from Denny, and various pieces and parts from everybody from Chris at Eagle Firearms to Wes at MSTN  and everybody in between (Talon Arms, Operation Parts, G&R Tactical, SKD, Bravo Company, JTAC, Brightflashlights, DSG Arms).
It's the best rifle I have ever owned, shoots 1/2 inch at 100 yards with high quality ammo, has been flawlessly reliable, has multiple optics and barrel options and is close to being as expensive as a semi-auto AR15 can be.
But the bottom line is that you'll get the most for your money on the EE.

If this sounds like an ad it is because, well, it is.  These guys (and girls) are enthusiasts just like us, don't settle for some pre-built overpriced under-optioned rifle at some gun shop, when you can have the best right here.

P.S. Many thanks for your service to our country.