I am interested
These are two very different animals. The MX135 was designed to be a surround processor first, with stereo capability second. The C45 is audio only. That means to get 5.1 out, you have to have 5.1 from the source in via 6 individual RCA jacks. C45 also has no video switching capability. The MX135 can take input via SPDIF or TOSLINK and will take video input from RCA, component and S-Video. Audio output is balanced or unbalanced while video output is component only.
Both are superior units and bullet-proof reliable. If you are going to do all of your 5.1 processing in your OPPO and run its video direct to your screen, the C45 is all you need. OTOH, the MX135 has some serious 5.1 processing capability. I've not auditioned the OPPO, so you will have to make your own call on whether the MX135 processor might sound better than the OPPO system. I can tell you that the processor system in my McIntosh MAC-3 works very well indeed, and it uses an older design than the MX135.
Good luck & happy listening!