Happen to have the ADS Pyro model, as well as the above dual enclosure. They are both very well made, and have never had a problem with either. Both (natively) compatible under Xp, of course. For comparison purposes, the ADS Pyro currently houses a Lite-on 52246s, while the dual enclosure contains two Western Digital 120GB SE drives (btw, the cable select method functions best with this particular enclosure).
If portability is a concern, the ADS model may have a slight advantage; however, speaking from personal experience, the Mapower enclosure is also quite portable. It readily fits into a small carry-on sized bag. On a number of occassions I have packed it on business trips if I am required to carry a large amount of data with me. The weight difference between the two is neglible.
The clear advantage of the ADS Pyro kit in this particular case is that it houses 5.25" devices as well. Above that, I would say personal preference is the deciding factor.