Zero is less powerfull than other pi, but it's consuming less, so battery can last longer.
I did not tried all the different systems on the pi zero! I think everything up to gameboy advance run very smoothly. N64 will probably be laggy (but I did not tried though)
buzz wrote:Actually pizero is clocked a bit higher than pi1 (original pi1 was 700mhz, zero is 1ghz)
But you could overclock it to 1GHZ pretty easily, the "game changer" is the RAM, doubled as in the 1st gen RaspBerry PI
Anyway, I'd say that everything up to 16-bit era can be emulated pretty well. Everything above this might be hard to emulate, but I will know more when i'll get my Zero and can stress it a bit!
Recalbox is really easy to install and work out of the box.
Retropie is easy to install too, little harded to configure, but have more "possibility" of customisation/configuration.
Atari 2600, NES, SNES, Genesis, 32X, Sega CD, Game Boy, Game Boy Color, Game Boy Advance, Game Gear, Turbografx-16 should all be playable on the Raspberry Pi Zero with no problems. Some MAME games should run fine, but I think CPS3 would lag. CPS2 should play with no issue though. Neo Geo is questionable, I think it would run with minor lag. PSX and N64 would both have some heavy lag.
I'm talking with facts, no assumptions or lucubrations. With Zero overclocked @1000 or Zero W:
- Sega 32X, Sega CD and Neo-Geo pocket laggy, forget.
- Forget N64, at all.
- PSX not at 100%, some titles very playable.
- All the Game Boys, GG, Master System, Genesis, NES, SNES and Turbografx at 60fps.
- MAME, FBA (CPS1, CPS2, CPS3, Cave) and Neo-Geo at 60fps.
buenas tardes 
(original pi1 was 700mhz, zero is 1ghz)