I've owned a VS2 stone, an SI2 stone, and my current I1 stone. All cut by the same cutter -- Paul Slegers of Crafted by Infinity Diamonds, all super-ideal, top-of-the-line cut quality. All three diamonds had equal sparkle -- and a lot of sparkle! But as others on this thread have mentioned, some inclusions, such as large clouds and/or large feathers, may affect the diamond's appearance and perhaps the diamond's sparkle. The clarity grade contains a wide variety of inclusions, and some people do not like certain types of inclusions but are okay with others. I do not like twinning wisps, but don't care about small feathers; others are the opposite. Some people prefer one larger inclusion that can be hidden under a prong. I prefer a greater number of small, clear inclusions that are dispersed. Some people are uncomfortable going below a certain clarity grade because they don't want to know the inclusions are there, even if they can't see them, or they are worried they may someday see them. Another thing to keep in mind -- the larger the diamond, the more likely you will be able to see the inclusions in some or all lighting conditions. My VS2 was a half-carat; my SI2 and I1 diamonds are 1 carat. So it really is personal preference. One thing for sure, if you are able to be flexible on clarity, you will save quite a bit of money and have more stones to choose from.
ETA: My comments refer to round brilliant cut stones. Fancy shapes, such as emerald cuts and princess cuts, may require you to go higher in clarity to ensure that the diamond is eye clean.