<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Jac747:
I ...am worried about my MP Visa miles. Isn't it somewhat of a drawback with the SPG card that you have to transfer in increments of 20,000 to get any bonus miles?</font>
You have answered your own question. With a MP Visa, you may have to worry about United. But with a SPG Amex, you don't have to worry because your miles aren't tied to one program.
As for the lack of bonus when transferring fewer than 20K ... Remember that there's nothing wrong with a 1:1 transfer - that's what most other cards give. Also, even without getting a bonus, you're still able to top off many program - a benefit few cards provide.