I'm about to replace my AR-15's upper. It's currently a flattop with an M68 mounted. I'm looking at RRA's Entry Tactical Upper Receiver, but I'd like to get something more flexible than a standard A2 front sight post. I figure if I get the A2 sight post I'm stuck with it, whereas one of the hi-speed rail block things, I can detach and re-attach a front sight at will. I have no real experience or knowledge of the different front sight assemblies RRA offers, and looking for stuff on Google doesn't turn up anything helpful.

RRA's options are:
- A2 Front Sight
- Weaver Gas Block
- Gas Block & Detachable Front Sight
- YHM Front Sight Gas Block Assembly

So, any recommendations? Any "stay the hell away from this!" experiences? Any help is appreciated! :)

For reference, this is the RRA website for the upper: http://www.rockriverarms.com/item-detail.cfm?ID=AR0840X&storeid=1&image=uentry07.gif&CFID=3655954&CFTOKEN=85329977