We do not recommend using a dedicated IP. Dedicated IP's only work if you carefully grow and monitor your sender score. The promise of being solely responsible for your sender score may sound good, until you do accidentally get blacklisted, or hit a spam trap. After which your sender score will be irrevocably damaged, and delivery becomes a real challenge. If you start out slowly and send a lot (as in millions), this is less of an issue.
At Flowmailer we do send millions (and millions) of emails per day. We also know how to keep our sender score excellent, because of monitoring. This means the sheer amount of email we send out grossly outweighs any damage a single sender can do. And even if one of our IP's should drop in score, we can simply send your e-mails via another IP in our range while we fix the problem.