I own a GTS and an MX (in my two machines). At 800x600x16, I notice no difference at all (although Quake reports higher frame rates for the GTS). However, going up to 1024x768x32, or even 1280x960x32, the difference becomes much more noticable. I think this is because the MX has a lower memory bandwidth, and fewer pipelines in the GPU (it can do less things at once). My understanding is the peformance stats go like this:
1. GeForce 2 Ultra
2. GeForce 2 GTS Pro
3. GeForce 2 GTS
4. GeForce 2 MX DDR
= GeForce 256 (the 256 is faster in older games, the MX in newer games)
6. GeForce 2 MX SDR
7. GeForce DDR
8. GeForce SDR
Many people don't realise the MX comes in several variants - there are versions (mostly from OEMs) with SDR memory, and others have DDR. Some have 64-bit memory bus and others have 128-bit. This explains the differences in price.
I'm quite new to this game so if anyone wants to dispute my ranking plz go ahead.