The ban prohibits having a treaded barrel so you are stuck with either a naked barrel or a muzzle brake vice a flash suppressor. The other item most commonly added is a collapsible stock. The bayonet lug isn't such a big deal to live without.

My first handful of AR’s were post ban then I found the need to expand into pre-ban rifles.

If the AR is going to be your primary defensive weapon, you will want the flash suppressor.  This is especially true if you are going with a short barrel, as less powder will be burned in the barrel and muzzle flash and blast will be greater.  Muzzle flash WILL blind you at night just as well as if someone shined a spotlight in your eyes.

Also, many prefer collapsable stocks on their carbines.  Though this is usually not *needed* (though can be good for kids or smaller-statured adults), they complete the carbine "package."

As Paul points out, the bayonet lug isn't terribly important or functional (even the military has little use for bayonets anymore), if you're getting a pre-ban, you might as well have it; the more you can annoy the anti's, the better.

Anyway, if you only see yourself plinking at the range, building a custom long-distance gun, have a pre-ban AR already, or have another suitable weapon as your "primary," then you'd probably be better off with a post-ban and the extra money.

-Troy