It's all going to be very personal, but for me learning to fly was hard enough without switching types. I did all my training on a C152 (although I flew several separate versions which were surprisingly different) and that was all my little brain could cope with - I had no desire to make things any more difficult than they already were!
Once I had my PPL I switched to a PA28, mainly becuase I wanted the four seats - my club didn't have any 172s so that wasn't an option at that time. So if you're finding the course easy, go for it. If you're finding it hard work, like I did, I would advise waiting. There's plenty of time later.