DIFFERENTS BETWEEN X5 and X3 you can see on the last picture.

Supports 99% of games from SEGA library.
64mbit (8mbyte) PSRAM.
56mbit (7mbyte) max ROM size.
128Kbyte Battery RAM. FAT32 file system supported.
Micro SD cards up to 32GB.
Fast loading (2-4 sec).
Built in audio playback. GYM, TFC.
Regular game battery back-up saves. Genesis, Mega-drive, Sega Master System, and 32X games supported.
Hardware MEGAKEY.
CD BIOS loading.
CD RAM cart feature.
Cheats feature.
Game Genie and Play-Action Replay format supported.