The real answer to this question depends on how long you intend on keeping that iPad 2. If you think you're going to hang onto it for several more years, you're going to have to deal with iOS 8. Maybe iOS 9. Since it runs slower on your more limited machine, you'll have to make some accommodations and changes to optimize its performance.
On the other hand, if you expect to be replacing this iPad sometime in the relatively near future, you might just as well stick to iOS 7, since it performs better on that particular machine. You can upgrade to iOS 8 or IOS 9 when you buy newer machine that already comes with it.