Before you upgrade you should evalute your brewing and if you will be converting to all grain in the future. IF you are going to start allgrain brewing id suggest buying a 16g kettle, as you can boil 5g batches with a lot more breathing room then a 10g kettle. This is of course my opinion, as i know some brewers on this forum who brew 5g batches in what i consider ludicrously small kettles.
That being said, why dont you just add a ball valve to you current kettle? All it requires is the ability to punch a hole in your kettle(step bit or hole punch work) and a weldless bulkhead with ball valve. easy peasy:)