I'd avoid mounting optics on the dust cover like you see with some of those Leapers/UTG products. It's not stable enough.
Mounting on the gas block is nice in terms of allowing a consistent cheek weld, but it also puts more weight on the front of your gun and puts the reticle further from your eye. It's also debateable whether the gas block cover is a stable mounting platform, but that's probably more a question of product quality.
I've been pleased with the side-mounted rail. It's quick-detachable if you get the right model (like K-Var's KV-04S) and very stable. With forward-mounting, you could have problems if the RDS dies on you and it doesn't co-witness the irons.