I personally have a 6hp (250 cc) Yard Machine rear tine tiller and I only do about 750 sq.ft. I would say that you would be MUCH MUCH happier with the bigger one. If you can afford it, I would say that going with alittle extra "oomphf" is far better than not enough. Better to know that half throttle is plenty than to open it wide open and not be able to complete the job.

1500 sq ft is going to be a bit much for a mini tiller.