MAME used to emulate arcade machines only and MESS was for console/handheld/computer machines.
Now MAME and MESS have merged and MAME can handle arcade + console + computer systems.
MESS emulator is no longer maintained (I think). Everything is on MAME now.
No, Emu Loader and EmuCon will not merge. EmuCon doesn't rely on an external games database, but Emu Loader does.
With EmuCon's games list show only game files you have.
Emu Loader lists all games MAME emulates, even those you don't have the game files for.
There's no way to merge both my frontends since their core is very different from each other.
You will be able to run all your console/computer games with Emu Loader (using MAME v0.162 and newer only), but EmuCon supports several different emulators for each system and it's way more flexible than Emu Loader.
Ciro Alfredo Consentino
EmuCon Home: http://emuloader.mameworld.info/emucon
MCM Plus: http://mcm.mameworld.info