Personally I use both.
The half mil is good for planning and the overview, but the quarter is much easier to navigate by whilst actually flying if low and reasonably slow.
Assuming you actually plot your route on the chart, rather than just use GPS. But even then I tend to use the memory map quarter mil on an Ipaq with a bluetooth GPS receiver as confirmation.
Of course if you are blatting along from VOR to VOR at 140kts then the half mil is fine and less of them to carry.