For choosing a validator to delegate and stake your tokens, nearly none. If you choose one of the validators with less rose staked, it may be replaced by another one, so you will not receive rewards, but if you use one of the first lets say 50 or 80 from the total 120 validators, you will do just fine. Your rose is just locked in your wallet, so there is no risk of losing it, and the chance of token slashing is little as I don't believe anyone wants to lose their tokens risking bad practices.
It is mostly personal preference at the moment of choosing, you can check their socials and their activity at oasis scan and see which one you like.
https://www.oasisscan.com/validators