I only own a generic spmp8000, not letcool and never had any dingoo too.
As for me, my device has 4.3" screen so when I considering of buying A320, I thought 2.8" is just too small. Still A320 is have better software and emulator support and I only will consider of buying it if ONLY I really want to get the best game experience. Since I'm not much of gamer, I don't want to waste much money on this. Maybe later if I really want to play it, but by that time maybe there're better device and SPMP8000 could be quite good by then.
I'll try to make comparison. Correct me if I'm mistaken.
1) 4.3" Generic SPMP8000 - Cheapest of all at $59 with free shipping, GBA and SNES emu sucks, big screen but widescreen at 4.3", 16mb ram.
2) 3.5" Letcool - Mid range price at $74+Free Ship, Dual-player support with gamepad, GBA and SNES emu sucks, middle size screen and good resolution for emulator, 32mb ram.
3) 2.8" A320 - Mid range price at $83+Free Ship, great support for software and emu, smaller size screen but good screen aspect for gaming, 32mb ram.
4) 2.8" A330 - Higher range price at $115+FreeShip, should work with Dingux, smaller size screen but good screen aspect for gaming, 64mb ram.
Also, you might want to consider some other thing like battery life, processor, digital camera, video codec and more.
If you budget allow you, just go for the best which is I don't know.