I have a TomTom ONE 2nd Edition, my sister just received a Garmin nuvi 200. They are currently selling for reasonably similar prices. Really, the maps and routing software (as well as the general user interface) will be similar for a maker's entire line; the exact features will vary. From using them both, I think I have a fairly decent idea as to their strengths and weaknesses.
Garmin-
Strengths: vastly superior maps and routing software (the main reason you buy the thing, right?).
Weaknesses: User interface is a little obtuse (hard to use, esp. while moving), does not come with computer cable or software (must buy cable and download the software), and the signal reception isn't quite as good as the TomTom.
Tomtom-
Strengths: excellent signal reception, comes with everything you need, great user interface.
Weaknesses: maps can be spotty in some areas (I bought the current 7.10 maps; they miss a change nearby that has been there for about 3-4 years now), routing algorithms aren't as efficient (might not take you the best route as often as Garmin, but nothing is perfect), short battery life (2 hours; be sure to keep it plugged in when in the car).
Between the two, the Garmin would be my choice due to its better maps and routing. After all, that's the primary reason to have a GPS; if it doesn't do that well (not that the TomTom sucks, mind you, the Garmin is just better), why bother? However, the TomTom ONE does a better job in usability and the side features. I don't feel poorly equipped with my TomTom and I wouldn't sell it to buy a Garmin, but if the Garmin nuvi 200 is within $50 of the TomTom ONE, get the Garmin.