... The fixed carry handle sight is problematic - you basically use it that way, and forget about mounting a scope at the proper height. It can't. Otherwise, most AR15 builders use the A4 flattop with rail, as it's far more versatile. You can mount the M4 carry handle sight for the retro look, or use flip ups, and then add a red dot, too. ...
I agree with Tirod. If you go with the A4 receiver you have a very flexible AR. You can still use a removable carry handle if you like the look or take it off and mount a flip up rear sight like the MBus2 which gives you room on the top rail for a red dot sight or scope. It won't be a true A1 or A2/A3 but with the M16A4 style carry handle it'll do what they do plus allow you to change your mind down the road.
Years ago I was very tempted to go with an A1 since it was much like the M16 I carried in Vietnam but now I'm glad I went with the A4 design instead.
Of course none of this means a thing if you really, really want the fixed carry handle.