Along the lines of what Neuro said, if you are upgrading to FF in the future, don't commit to one prime. When I bought my first dSLR it was a crop and the primes I got use to performed completely different when I upgraded to a full frame. I was all disoriented because they were completely different on FF. More than likely with a zoom you will end up shooting at a certain (ball park) cropping and then when you get the FF, times that sweet spot by 1.6 then by the corresponding prime lens.