Are you asking which is faster or which will blow up faster, The RXP's had supercharger issues the Yamaha's had power valve issues, both usually mean a trip into a damn expensive engine, truefully its pretty much a toss up on those two machines.
to go along with the above... I don't know much about the Yamaha but on the Seadoo side of the equation.
The speed should be comparable the Yamaha might even be faster like said above... For me Seadoo's especially the newer 4 stroke 4-Tec engines have been extremely reliable machines if proper care and routine maintenance is completed. (i.e. oil changes, winterization, spark plugs, etc) even the supercharged ones are reliable once they are updated (From 2003-2006 they came with ceramic supercharger clutch washers that would break around 70 hrs of use or less... sometimes alot less so the they need to be upgraded to heavy duty steel washers) In 2007 Rotax updated the supercharger and it now comes factory with steel washers...
The supercharger is not an issue on a 07 Seadoo RXP 155 b/c the engine is naturally aspirated (no supercharger)... The 155 Rotax 4-Tec is the same engine as the 215 minus the supercharger and intercooler that was made available in all the lower end models when they eliminated their 2-stroke engines completely...
To make the poor folk that could not afford the 215 but still wanted to have the RXP name happy they stripped it down and called it the RXP 155...
Model Info:
http://www.sea-doo.net/units/2007/2007_RXP Spec Sheet.pdf
I think the Rotax (Seadoo) is easier engine to work on b/c it is all over head valve and a much more basic design IMHO...
