GX is generally the preferred Gen/SCD/SMS/GG emulator even on high powered PCs for being balanced between accuracy and speed. It's touted as having 100% compatibility
with all the system's libraries it emulates. The sound emulation is particularly good; just as good as Kega Fusion's as far as I can tell. Exodus is meant to be more accurate, but it's still a work in progress and supposedly still slow on powerful PCs.
Picodrive is a fine speed emulator, but since the Zero can run most games through GX full speed it seems a waste of the system's potential to use. For a specific, easily noticeable accuracy test, play Sonic 2. The sound effect for the signpost spin at the end of the stage sounds wrong (very harsh) in Picodrive but correct in GX.
I still accidentally exit mid game by hitting select with the bottom of my thumb sometimes, so if you ever do develop this port more that should be one of the first things you change