Mac minis never had a G5 processor. G4 then Intel chips. The iMac G5 is prone to power/video issues. If the G5 tower is one of the liquid cooled model they are prone to leak coolant on the logic board. The Intel mini is a good deal, as is the G4 mini.
$700 for all is a pretty good deal though. I'll see if I can find the info on the iMac G5. It all depends on the model.
Here is the link to the Apple support page.
http://support.apple.com/kb/HT2173?viewlocale=en_USTry to get the serial number and you can check the model on the Support page at apple.com
If it's the ambient light sensor model I would pass on it. Otherwise, it's a good machine.
BTW - only the Intel machine will run Snow Leopard(10.6) the others will run Leopard(10.5).