They would be relatively evenly-matched. Were the circumstances of his acquistion the same as that of the other aliens, Snare-oh would be the Omnitrix's sample of a genetically penultimate Thep Khufan, giving him the physical advantage; Kuphulu does seem a bit puny for a Thep Khufan. However, Ben scanned Snare-oh from the Mummy, so it depends on the Mummy's physical properties.
Kuphulu would have a lot more experience with his abilities than Snare-oh would, however; Derrick J. Wyatt and Ben himself have both mentioned that Ben has little knowledge of the powers of a Thep Khufan, whereas Kuphulu would've had years to perfect his skills. It would be like having a karate master fighting against a whitebelt. So in my opinion, Kuphulu would have the upper hand.
I think Kuphulu would have a better chance of victory, as he has had apparently many more years of experience as a Thep Khufan under his belt, while Ben seems to still be getting used to Snare-oh, but he is catching on rather quickly. From the randomizer transformation to knowing little to nothing about the form, he managed to use Snare-oh pretty well afterwards and rather quickly, such as stopping the stampede in Undertown to defeating Psyhpon and Vilgax's squid creatures.