I think if you’re riding around a trail park with armoured single track, carefully crafted berms, the odd jump, and the accent on interesting downhill bits, you’ll want a bike with lots of suspension and the majority of them are 26″. You’re never far from anywhere, and there’s plenty of other riders if you need help, so a complex bike and a macho approach to riding are more fun.
My riding is done in the Highlands, usually isolated, tracks can be anything, there’s usually no-one else around, and the bike may have to be shouldered for peat bogs or lifted over fences. I have found that the lighter the bike the better, and the less bits to go wrong the better. Because I tend to be on my own a conservative attitude to riding is necessary – don’t want to bring out Mountain Rescue because I’ve been stupid.
Thus a rigid 29er single speed works best for me. Just about unbreakable and light. I also do 24 hour solo races and again that 29er does the job, all I have to worry about is riding not maintenance.