They're both good guns (ok, so we shot Ions last year and are shooting Promasters this year, so I'm bound to say that.) Both are very straightforward designs internally, so if you look after them, you shouldn't have any trouble. With both guns, 9 times out of 10 if there's a problem you just need to change the battery.
My preference is for the Promaster, it's a well-built gun which has evolved from a long line of ICD products (Bushmaster, B2K, BKO), but the usual advice is true: try both and see which you prefer the feel of.