I would buy whichever one is cheaper, assuming both are equivalent in the sense of being switchable HPS/MH, and dimmable ... if you want that.
Both are high quality digital ballasts, and really the only two things that should affect your choice are the price, and the warrantys offered.
I may be wrong, and if so i'm sure i'll quickly be corrected (happily, as I like learning ) by the great growers on here ... but ... I don't think I am