I have been using Delorme about 8 years, right now I use DeLorme Street Atlas USA 2005 Plus. I have used MS streets, I think it was the 2003 version.
Both do a good job of telling where you are, but the maps aren't always right.
Both can give you a route from point a to point b. Both will have some goofy things in the calculated route at times, but my experience was MS streets did some really crazy things and Delorme's were relatively minor. (On a trip from AZ to TX MS Streets wanted me to cross the border into Mexico instead of take I-10 which was both shorter and faster)
Delorme has several products that can track with a GPS, including Street Atlas USA, Street Atlas USA Plus, and Topo USA. Even the most basic Delorme program has a lot more features than the MS Streets version I tried, though you may not need or use them.
Things that the basic Delorme does that I don't think MS had in 2003;
Give spoken directions
Take spoken commands from you
Have multiple routes shown at once, and selectively show / hide individual ones
have multiple draw files shown at once, and selectively show / hide individual ones (I can have a flying J layer, a walmart layer, a microbrewery layer etc, and just show the things I am currently interested in seeing)
can import a (properly formatted) address book file and have the places show on the map.
The Plus version adds:
XData, an interactive linking to an excel spreadsheet
a big phone database
Topo version;
does routing but not as full featured as in Street Atlas
has much more detailed maps with elevation detail
Can show a vertical profile of a route
has a 3D mode that makes it easy to visualize the terrain
Another program that just came to the US about a year ago is Route 66. It's an ok program but basic features similar to MS Streets. They have both windows and MAc versions though.
So I like Delorme for the features, while any of these 3 will tell you where you are, none of them will guarantee you won't go down a dead end street.