with the naked eye you're not gonna see much (if any) of a difference between VS2 and SI1. If you do, it's likely just in your head. the cut is going to make a bigger difference for the sparkliness, methinks.
depending on how big you're looking at to start with, 0.25 carats can make a big difference (i.e. 0.5 vs 0.25) but as you get to the larger that extra 0.25 becomes much less noticeable (i.e. 1.75 vs 2).
if it was me, i'd go with the larger stone.
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