the GPS lock will work similar regardless of Nav app ... many have confirmed that TomTom's lock is quick and holds well.
don't forget about Navigon - most report the best overall user interface, good routing and strong features ... they will discount soon; they've always been lower priced than TomTom ... and they offer fixed price traffic (from INRIX, see free App for sample of traffic accuracy) ... and they are providing a free update w/in the next couple of days that includes Google Local Search which is by far the best POI database ... in comparison, TomTom charges $9.99/mo for live traffic, google search, and fuel (see TomTom Services) on their other devices
the one drawback is that on a iPhone 3G, Navigon is a bit laggy when using the menus; road navigation is fine but moving through app is slow ... supposedly this may improve w/ next version, but for now it can be slow
just providing you add'l info for your decision ... good luck
are you able to look at the route details while you are navigating using Navigon? friend has the app and we couldn't figure out how to look at the route details while navigating.