What Terry and Marvin said. But let me see if I can add something to the discussion.
Take a look at this thread comparing nroute and Mobile PC
If you have looked at the Garmin Nuvi handhelds, Mobile PC looks and behaves very much like it. It's a lot like turning your laptop into a Nuvi. If you have a notebook with a touch screen, Mobile PC will use it and that makes it even more like a Nuvi.
That means Mobile PC has very similar strengths and limitations as a handheld. It's super for navigation - blows the socks off Streets&Trips and Street Atlas, in my opinion - but it's not so hot for trip planning. The maps have everything you need (same database, from Navteq, as the maps in Streets&Trips). Mobile PC just doesn't have the planning functions because it's pretty much a clone of Garmin's popular handheld personal navigation software.
You didn't say what maps you have been using on nroute. You could use those for your trip planning, using MapSource. And then, using Mapsource, you can transfer any routes and waypoints you have created over to Mobile PC (MapSource thinks Mobile PC is just another handheld device). MapSource lets you print maps, too, if you want. It's not Streets&Trips but it's pretty decent for trip planning. Much better than Mobile PC, in my opinion.
As Terry said, if you like Streets&Trips for trip planning and want to be able to transfer routes and stuff from the desktop to the laptop for travel, you pretty much need to have Streets&Trips on both (which you can do, no problem).
If you want a Garmin solution you need MapSource and maps for the trip planning and Mobile PC on the laptop for navigation.
If you have different generation maps on the desktop (with either Garmin or Microsoft solutions) you will need to recalculate the routes on the navigation computer (assuming it has the most current maps) to see if you still like the routing it's giving you.
Take a look through the Garmin section
on here for some of the threads of interest before you make a decision. There is, unfortunately, no clearcut answer to your question "What is best for me?". You can see that's quite a difficult question because we aren't you.
Oh yeah, for what it's worth I have Streets&Trips 2008. I bought it strictly for the GPS receiver, although I have experimented with Streets&Trips, too. The receiver works great. I have a Garmin iQue 3600 as my main nav device and the Streets&Trips (Pharos) receiver blows it away for speed of getting a lock and sensitivity in tough situations. I'm using the Streets&Trips receiver with nroute and Mobile PC, not so much with Streets&Trips. I'm still kind of a hardcore Garmin guy (just so you'll understand that there is lots of bias here ).