The MX has half the memory bandwidth, either 128 bit SDR or 64 bit DDR, while the GTS has 128 bit DDR, also half the pixel pipelines.
I honestly don't see a reason to endorse the MX....I mean, the Radeon 32 whips it around in performance and is only a little more expensive...so I'd say either the Radeon 32, the GTS, or the V5. What games do you play?