Hi Nige, what boat are you thinking of putting the motor in, I did a lot of testing a bit ago with the graupner 7.2, 9.6 and 12v, and the 12v neodyme all 700bb turbo, so perhaps I can offer some of my findings to help you. I run a 9.6v with 2:1 gearbox in a 3 foot Aerokits 1/16 scale Fireboat. Run it on 19.2v (two 9.6v nimh packs, 4600mah). When changing to a 7.2v motor with same set up and packs, the gps speed was no real difference, but now the motor is running over voltage, and hot. The 12v motor was useless with same set up, with the gearbox. Different story with all motors on direct drive, the 12v on 19.2v was much quicker, ran cooler and longer.
Back to the two motors you want to know about, the 9.6v runs a bit cooler, but very little in it, if you run the 7.2 on 7.2v, and the 9.6v on 9.6v, both battery packs with same c rating and mah, I doubt there will be much difference, but the 7.2v on the 9.6v pack will be quicker , you have to get them working a bit if you want speed, but then run time suffers, now the neodyme is a different kettle of fish, this was by far the fastest on same set ups, but it costs more money.
All the results are documented on "whats my set up" in the Fireboat website
, hope that helps