If you are watching videos, and the videos are playing correctly, then it really doesn't matter which format you're using in that regard. HTML 5 is a newer format than Flash, but whichever works for you should be just fine. Flash has a bad reputation for being buggy and riddled with security flaws, and can be a nuisance when it comes to frequency of updates. HTML 5 support isn't exactly fully implemented in a great manner yet, so I would say it's buggy in it's own way, but in a manner that is different than Flash. Over time, dependence on Flash is supposed to diminish while HTML 5 displaces it.