I'm making the same decision right now. I'm picking up my new MX on Monday afternoon. I ordered it with the stock 20" slipstream wheels (silver). Tsportline has the 20" TST wheels on sale this weekend for $1920 (black color) including free shipping. The cost to powder coat my stock 20" wheels (here in Denver) is about $500. I actually like the TST wheels slightly more than the slipstream wheels, but not enough to justify spending almost $1500 more to get them (and I don't believe one wheel is functionally better than the other).
I did consider buying the TST's and holding on to the stock slipstream's and possibly put snow tires on them, but the MX is so good in snow (better than my current Subaru Outback), that in reality I'll never put snow tires on them - even living in Denver Colorado.