If you like a G6 with 512mb go for it, it has almost perfect GBA support. If you don't care about GBA that much and want a lot more storage capacity and better support for slower flash memories, go for a SC Lite. Nowadays the games tend to be 128mb+, might be 256mb coming before the current year ends so I would never go for a G6 since it costs more and has no extra storage capacity.
Telling that SC lags is pure crap, M3 has the same problem if you buy a regular Kingston/Sandisk card. The Castlevania intro is just flawless with a regular sandisk if you have a SC Lite.
If you want better GBA support and you are willing to pay like 30-40% extra for that and some slow menus go for an M3 Lite. Otherwise I would go for a SC Lite.