I use neither. Instead, I use SkyCharts Pro. It has all the charts, with geo-referenced enroute charts, but not all the frills that you get with WingX or Foreflight. It also only costs $20/year. If this recent fiasco with the FAA and their charting policies causes SkyCharts Pro to go out of business, I'll probably get Foreflight.
As far as memory usage goes, I got the 64G and have plenty right now. Still only have a smattering of my pictures loaded, but all my music is there. With that, my apps, and roughly the eastern half of the country's charts in cache, I'm currently at about half on my storage capacity. Keep in mind that you'll probably find you'll use the iPad for more than just aviation stuff (despite what you say now).