I also second the FC Twin, here's a review for it:
http://www.rfgeneration.c...-as-purists-claim-354.phpThe NEX is a nice system with a "cool" factor, but it commands a greater price and doesn't have the dual NES/SNES capability that makes the Twin a great space saver and double duty machine.
The Retro Duo appears to have solved many of the compatibility problems of the Twin and it's predecessors, but usually at twice the price of a Twin, but also $10-20 less than a NEX.
So here's the breakdown:
FC Twin - cheaper than the others, can buy it just about anywhere
Retro Duo - a bit more expensive, slightly better compatibility, S-video
Generation NEX - well designed, wireless controllers available, most expensive