From a security stand point, we don't store your private key. If you have 500+ steem power, then you can stick with steemvoter to get the 25 free slots, otherwise @steemdunk offers more (20 free, regardless of power). As steemdunk is payed annually, you can get quite a bit more slots for cheaper (starting at $0.41 SBD/month or $5 annually). There is a new feature being released soon, and that allows restricting max upvotes per day per author to save VP, which is a much requested feature. Ultimately, it's up to the user to decide which service to use. Thank you for your interest.