A4 is a flattop. You can mount optics directly to it.
A2 is the carryhandle that has iron sights. Typically, you don't mount optics to it, but there are mounts that do that.
RRA is praised by many people. I think they're ok. I don't think you'd be making a mistake with them. The best thing they've got going is their trigger. I'm not certain if they're chrome-lined by default, that might be an option. They probably mean chrome-moly, which is just the steel they use.
I'd take a look at Armalite too. They've got chrome-lined barrels and a mid-length lineup. Their fit and finish and overall quality is excellent.
Other good choices would be Colt and Stag. Although, the RRA is probably the best value in that it's the most gun for the price.