well the reaper is has higher memory settings then the platinum does and it also has a better heatsink/ cooling device
but first of all there is 2 types of reaper,
OCZ Reaper HPC Edition 2GB (2 x 1GB) OCZ2RPR8002GK at $55 us after rebate
which has timeings of
Timing 4-4-4-15 2T
Voltage 2.1V
and OCZ Reaper HPC 2GB (2 x 1GB) OCZ2RPR800C32GK at $97 us
Timing EPP 3-4-4-15
Voltage 2.1V
naturally the lower timing model OCZ2RPR800C32GK is slightly faster, but also double the price, next we take a look at platinum version
OCZ Platinum Revision 2 2GB (2 x 1GB) OCZ2P800R22GK at $38 us after rebate
Timing 4-4-4-15
Voltage 1.9V - 2.1V
now the reaper has lower timings and a better heatsink, personally i would go with the $55 ocz reaper ram OCZ2RPR8002GK, those timeings are good, and the heatsink is good, ive recently heard that someone had the platnium ram, and the heatsinks got to hot and warped, and ive heard that the reaper heatsinks are a well designed piece that cool efficently,
now it is for you to choose which you like and how much money you want to spend