I think it's worth repeating that the maps that come preloaded in Mobile PC cannot be used in Mapsource.
Further to that, Mapsource is no good without detailed maps. Even a basemap, which can be downloaded for free, is of little value for detailed planning because it only covers major highways and roads. It has little street data, no address data and no points of interest.
Although you can download and install Mapsource for free, you would need to buy a duplicate set of maps and that's actually more expensive than buying Mobile PC complete with built-in maps.
And it's another interface to learn. It's completely different, again, from either Streets&Trips or Mobile PC.
I think, for most practical purposes, the routing flexibility built into Mobile PC is more than up to the task for trip planning. And it's an excellent navigation program. For anyone considering switching from another laptop program that combines trip planning and vehicle navigation, I would recommend looking at Mobile PC on its own merits. If its features and functions match your needs, go for it. If not, consider something else.
The main difference between Mobile PC and Streets&Trips is that S&T began life as a trip planner and has had navigation sort of tacked on. At least that's my impression ... it's navigation features leave me cold. Whereas Mobile PC began life as a navigation program and its routing functions are an integral part of that. They happen to be quite good, from Garmin's long history of routing and navigation. But Mobile PC has evolved out of the handheld devices and it's primary intent is to put a Nuvi on your laptop. As such, S&T users will find it a little short in some of the trip planning features that don't have any direct connection to navigation.
A better option for the folks considering Garmin and who want a little richer trip planning but still want excellent vehicle navigation would be to buy a Nuvi model that has the desired features. The Nuvi model lineup covers a huge range of prices and feature sets. Buy only what you need. That will give you great vehicle navigation and also personal navigation in a device that is also reasonable to use as a handheld.
Add Mapsource on your PC to the mix and you have a powerful combination. Mapsource is an excellent trip planning program with most of the features of S&T and some unique ones of its own. As I mentioned above, it will need maps. If you buy a Nuvi, when you first set it up you connect it to your PC with a USB cable and it allows you to have it check Garmin's web site for updates to its internal software. At the same time, if you allow it to, it will download a copy to your PC of whatever map product is preloaded in it. You can use those maps in Mapsource.
You can create routes and waypoints ("Favorites" by any other name) in Mapsource and download them to the Nuvi. You can upload track files from the Nuvi to view in Mapsource. You can also view all these things in Google Earth, without any file conversion, via a menu option in Mapsource.
With the Nuvi and Mapsource alternative you haven't completely lost the ability to navigate with your laptop occasionally. There is another Garmin program that you can download, called "nroute", that has the same interface as Mapsource and uses any maps that are loaded in Mapsource and gives you all the vehicle navigation functions of any Garmin navigation device. It's not as pretty a display as a Nuvi or Mobile PC but it works fine.
if that all sounds complicated. To do all the things I mentioned, it is. But we're here to .
It's actually quite simple to do the basic stuff. A Nuvi is dead simple to learn and use for navigation and Mapsource is fairly easy to use for basic trip planning. All the rest of the stuff is completely optional, depending upon your pain threshold.