Depends which systems you wish to emulate? The B+ will emulate most 16 bit and older consoles/systems no problem apart from SNES which has some issues on certain games (mainly super fx games) MAME/FBA also runs well generally. Anything after 16 bit era is hit and miss but performance gets an instant boost on the new Pi 2 according to members on this forum (don’t own a Pi 2 yet myself) which makes previously unplayable games playable. If you can get a good deal on the B+ and are aware of it’s limitations it’s a good starting point as there is plenty of info on this forum to get you up and running. The Pi 2 is the same form factor as the B+ so any cases you purchase will fit the Pi 2.
Lastly it’s important to get a decent micro SD card (Samsung class 6 are reliable in my experience) and a 2A power supply, this applies to both models.