I have no idea when to take snapshot. But I recommend that you sell your xrp after taking a snapshot. Because I think it will dump. If you want to keep xrp in your hand later, you can collect it after it falls. But if you want to make a profit on both sparktoken and xrp, I suggest you sell it.
That would be wiser advice to make a profit from this opportunity. Because after the snapshot people would going to dump it very hard. So selling spark token along with XRP would be a wiser move at the moment for the XRP holders and after the dump, if you should have to buy it back then you could do it, and so selling it after a certain time later can benefit you.