I've been using both side by side (VFR) on an iPad Mini 5 for several months, and have found pros and cons to each. Listed below are the items most important to me.
Foreflight (Basic Plus) has more refined graphics, full voice alerts, and a nice logbook function. Oddly, Basic Plus includes IFR charts, but you have to buy the $200 version to get terrain and obstacle data (and synthetic vision) I find this rather strange since terrain and obstactle data are arguably more important for VFR.
iFly (VFR) has terrain and obstacle data included (and synthetic vision), has very clear, high-contrast graphics, is much more customizable, is more intuitive, and has a fuel timer. No logbook however. It also doesn't have full voice alerts.
Both have rather primitive weather briefing functions. iFly relies entirely on raw text. Foreflight also include graphics, but accessing them is surprisingly cumbersome. Weathermeister.com is much better than either of them (but requires another subscription).
For me iFly is the clear overall winner in this comparison, although I'm hoping the next version will include more voice alerts and a logbook function. Maybe even an improved weather briefing function?