This really depends on the terrain but I doubt a cross bike is the answer. I race cross, to which a cross bike is ideally suited (obviously) but I've also done the Hell of North Cotswolds on it and I doubt I'd ever use a cross bike for that event again. It copes with the climbs just fine but when it comes to the downhills everyone on mountain bikes just flies past. The lack of any suspension means that you get seriously battered on the rough stuff and, of course, the brakes are cr*p. After a couple of hours it just hurts.