Well its cheaper as its specific for that scenario!
MST200 does not need any controller... it has their own OS....
Now the other option i just posted you will be needing the 3200MX
Now take in mind 3200MX
if you got a 3200 alone then you will need a mem upgrade package to upgrade it to the lastest firmware tnat is needed to support the ap 274/275
If you dontwant to buy the mem upgrade then you can use the AP 274/275 in instant mode(but it still not out the instant mode)
It will be out in a few months i bealive.(The instnat mode) The controller mode is already out
Cheers
Carlos