I chose to go with SI1 stones for the pave on my ering, which I should have in my hand by next week/pictures. We were working on an extremely tight budget and SI1 allowed us to get the price point where we needed it.
As far as sparkle goes, yes, it is possible for an inclusion to impair a stone's light return, especially when you're talking tiny little stones. Realize, however, that the average ring on the market uses SI or worse quality sidestones and melee. Go by a local big retailer (Zales, Kay, Jared, whatever you have nearby) and check out their pave engagement rings - those are overwhelmingly going to have SI sidestones and can give you an idea of what they look like. I think MoCo/AD standardly uses SI/G diamonds in their melee, so you can look at some of the pave rings that their customers have posted. I think the average person won't tell a difference.
Is this a ring you're having made by a board vendor or one you're ordering from an overseas maker like LOGR? I've heard (though not actually purchased from them) that with LOGR especially it seems like the "SI" stones are really more like "I" and cloudy and a lot of people have been unhappy with their recent purchases unless they upgrade. It really just depends on the jeweler, I think, and how accurate their grading is.