I don't carry my Shield 9 very often (I prefer my M&P 40 compact), but when I do, I carry 8+1, because I want to maximize the number of rounds I have available. (As many have written on the Forum, no one who has ever had to defend themselves has ever said, "I wish I'd carried fewer rounds.") I also have wide hands, so I prefer the larger grip.
However, that's me. What is best for you depends on a number of factors, such as how well you shoot your Shield with each grip length, how well each grip length fits your hands, how easy it is for you to retrieve your Shield from your preferred carry position with each magazine, how comfortably your Shield carries with the 7-rounder vs. the 8-rounder, etc. You can decide how best to carry by using the old prescription for getting to Carnegie Hall: Practice, Practice, Practice. With both magazines.
Good Shooting!