If it comes down to a "MUST sell one of them" situation I would sell the A500. There's so many reasons why but I won't bother you with that.
If it was me I would probably keep both and shaft the A3000 though... it's not nearly as rare as 030 cards for A500/600 and with a 040/40 in it I'm thinking heat issues and an uppgrade to 060 hanging in the air. Also you have an A4000 and an A1200 both with 060/50 so another "powerhouse" probably isn't realy needed.
What might be a good idea is to write down as many reasons as you can think of for keeping every machine, then read it and try justify keeping every machine. I currently only hve 1 machine that I can't truely justify keeping and that's my A500, rest I have reasons for keeping if ever so slightly.
But most of all... follow your heart and ask yourself what hurt the least to get ridd off.
