A GTX 285 with only 512 MB will not work for MPE acceleration.
Actually, Bill, the OP meant "512-bit DDR3" for the GTX 285. There have never been any GTX 285 cards with only 512MB of RAM. Most GTX 285 cards came with 1GB of RAM, while a few offered 2GB.
In this case, between 1GB versions of those two GPUs, definitely the GTX 285: You did a test with both of those CPUs some time ago with CS5.5 and an i7-2600K overclocked to 4.4GHz with 16GB of RAM. The biggest difference is in the MPEG-2 DVD test of the PPBM5 benchmark: The GTX 285 completed this portion of the test in 117 seconds while the GTX 550 Ti needed all of 146 seconds. This is in large part due to the higher 159 GB/s bandwidth of the GTX 285's 512-bit DDR3 memory compared to only 98.4 GB/s for the 192-bit DDR5 memory on the GTX 550 Ti. However, both of those GPUs have fewer than 384 CUDA cores (240 for the GTX 285, 192 for the GTX 550 Ti), so expect both of those GPUs to be the bottleneck in faster PCs.