I would keep the balances in BTC if you can, as you generally don't want bettors to think "I would have been better off keeping my BTC instead of betting". Usually when I use a book that keeps USD/EUR balances (which is very rare), I would deposit, place the bet, and quickly withdraw if I won so that price volatility would hopefully have very little affect. With a very expensive 0.5%/1.5% withdraw fee (seriously, this doesn't really make any sense), I would almost certainly never play at your book. However, there's probably many people who do like keeping balances in fiat, so why not make it an option? Is there a specific reason you have to decide on one currency or the other?