Other's will probably chime in on this, but I do not think a slightly oversized frame will be a problem for you. The sloping top tube frames are intended to fit a variety of sizes, and you could probably alter the bike by swapping out different sized stems to fit you properly (much cheaper than buying a new bike in a couple of years).

As far as what model to get, I say get what you like and can afford. I ride with people on rockhoppers, and they are great bikes. I also ride with someone on a hardrock, and it is a great bike too. You should ride them both if you can, and see which one you like best.