Hi CS,
If you already have Mapsource it's way better for trip planning. As Terry said, Mobile PC is designed as a PC version of the Nuvi .. designed to try to optimize the in-vehicle navigation experience, including touchscreen support. That inevitably meant tradeoffs that reduce its usefulness for trip planning. In Garmin's engineers' defense, they have included all the functionality you need. It just happens to be a couple of levels down in the interface due to the default (and correct, I think) assumption that the vast majority of users want to search and/or navigate starting from where you are, not somewhere else.
Good planning tools, like Mapsource and Streets&Trips start from the other assumption: that you could be wanting to search or navigate starting from pretty much any arbitrary point. This really affects the nav functions of Streets&Trips. Garmin just took the nav functions out of Mapsource rather than try to improve them.
Frankly, the iQue isn't as great as you make it sound in this particular regard. My personal favourite is still my iQue 3600 - the original. I like is so much that I've tracked down and purchased some backup units so if my original ever dies I've got replacements on hot standby. I now own my original, one used backup, one new backup and one new 3600A that I'm quite taken with and might make my prime unit (I love the aviation navigation screens!!).
I love the iQue 3600 to pieces but the iQue (all models, because it's essentially the same Que software that Terry mentioned above) is somewhat biased towards searching and navigating from your current location (actually the last location it got from the GPS) rather than from some arbitrary starting point because it was also designed as more a nav device than a planning tool.
You still have to fake it out to force it to start the search or plot a route starting from somewhere else. It's just that you already know how to do that on the iQue. Arguably it might be a bit easier on the iQue, but not a whole lot. Mostly you just need to know how. Once you know how, it's not a big thing on either the Que software or the Mobile (PC/XT) software.
In my opinion.
...ken...