I love my Area451. While Steven of smartieparts.com (who now has the controlling interest in Area 451 Parts, Ltd.) installed my cruise control personally, I helped install one in a friend's smart. I really enjoyed it. I like the thing so much, I put together the LED kit for the instrument cluster which lets you slip a light cleanly in behind the otherwise vacant CC icon already on the panel, sharing the joy with a few dozen other owners.
Tech support is also available at the forum.area451parts.com as well as from Steven at SmartieParts. There's even
free shipping in August!
The MDC has features the Area451 doesn't (sprint booster type features and speed increment/decrement).
The installs are roughly equivalent in complexity. If done
properly they should both be reliable installs.
While I've heard of more issues with the MDC unit installs (bad cable wires or otherwise faulty units) the exposure might be greater because of the number of units sold; I don't have a comparison of the two.
The Area451 was the first, designed from the ground up for the smart 451 model. The MDC unit came later for the 451 but had a long history selling a different design for the 450 model.
You'll be happy with either. I'd recommend what I know and love: the Area451.