Super-easy decision - the Mini is a dual-core Intel, and the G5 is a single-core PowerPC...
dual core or dual CPU is just so much faster, and less likely to bog down when you try to switch tasks, or do several things at once, it's ridiculous.
The G5's are on their way out... the last ones produced are pretty good, but the early ones are past their prime.
The current Mini is a fine little machine. Fast, slick, powerful, and other than the lack of a video card, it's as good as (or better than) the iMac in every way...