I only used Garmin Mobile XT on my P990 (UIQ3 based phone). I liked that it had very good maps, but the software had many bugs, crashed a lot, and was very slow, and it lacked a lot of features a navigation software should not lack (it could only show the estimated time of arrival, but it couldn't show the current time, the distance left, etc.) But, as I said, that was the UIQ3 version. TomTom on UIQ3 was very fast and stable, but lacked good maps. The software was better too, although Garmin XT had some very nice features too.
Now I'm using iGO 8, mostly because my phone was shipped with it, and because it has the best Hungarian coverage (it was made by a hungarian company after all
). The best thing I like about it that Top-Map (the map company, which is part of NNG, the company behing iGo) has an online map-bug-issue tracker, where you can send issues with your current map. iGo has also US maps, so you might check it too.
But between TT and GXT I would prefer GXT. Maybe the WinMo version is better than the UIQ version was.