The primary benefit of the dedicated iFly hardware is that it has a display that's easier to read in direct sunlight than any tablet.
Aside from that, the tablets have almost every advantage: Generally thinner/lighter, long-lasting internal battery, can be used for many purposes besides flying apps, decent units can be found for relatively low cost (especially in Android flavors). As of today, some iFly features are only available on phone/tablets, not on the 740 (RealPlan, for instance).
Most iFly users are using the software on a tablet device these days. I've owned a 700, then traded that in for a 720. I still have the 720, but I also have two Android tablets, and when I fly those two devices go on my yokes and the 720 stays in my flight bag as a backup.