Greetings,
This one's a tough one.

I think SCSI I Harddisk on the A500 is harder to find these days, I think you've keep the A600.
In terms of repairability, the A500 is above the surface mounted A600, you'd keep the A500.
Tell us which satisfy your cravings more? The more retro style(A500) or compactness(A600)? I think it depends on your taste here, AMIGAZ. What are you after?
Regards,
GiZz72