You've already done what most will tell you to do...try them out yourself. Pick whichever feels best for you, and whichever feels the safest. I have no problem with the P99 having no safeties, as I don't have any small children around or anything of that sort to inadvertently pull the trigger. Also, with a hammer fired gun, it makes sense to have the hammer cocked and a thumb safety engaged since a lot are single action only (I believe the USP is DA, however). But with the P99 being striker fired, it requires no external safety. All of the safeties are internal, and all you need to do is pull the trigger in order to fire a round. If you do not want to fire the gun, don't ahve your finger on the trigger. Don't think that because the P99 has no user enabled safeties that it is an unsafe gun.
As you've already stated, the P99 feels great. That's about all I can say about it...it's hands down the best gun I've ever held in my hands in terms of feel. I only had the chance to shoot it in .40, which was a shame, since I'm not too fond of that round. Had it been 9mm I probably would've bought it right there.
The safety issue is something that you will have to decide on your own. The P99 is perfectly safe as long as you're not putting your finger on the trigger all the time. Think of it like a Glock...just without the external trigger safety.