I hope this is helpful for you... I've written a bit about some of my own experiences.TomTom XXL 540S ReviewGarmin nüvi
I am currently using the Garmin 3790T. I like it. But, it plots some stupid routes sometimes.
Now, I must say that in retrospect, I was harsh on the TomTom.
The TomTom actually outperforms the Garmin in several areas. Generally, it plots better routes than the Garmin.
Still, the slowness of the TomTom and it's inability to plot some routes and it's confusing interface make it unsuitable for use by itself for me. I find it dangerous in some situations.
Now that I have several months of usage with both the TomTom XXL 540S and the Garmin 3790T, I can say that I think one of the problems is the actual map data. It seems like it's just far inferior to the map data you get in North America. This kind of makes sense as Australia is a smaller market, so I can see less effort being put in there. I would then forgive the TomTom and the Garmin for some of their idiocy in route planning.
In North America, I used 2 GPS devices: an SUV built in model, and a Garmin. Both were spot on in all navigation with good route planning.
I've used a Garmin in Malaysia with free map data. It was difficult to get used to and difficult to use because the map itself and the layout of Kuala Lumpur are simply complete disasters. If you've ever driven in KL, it's a big spaghetti disaster on the highways and freeways. You end up with 5 turn-offs on the left all within 100 m or so (literally), and can't react in time to read the map properly and look where you are going -- you miss your turn and take the wrong one. I don't really blame the Garmin unit for that.
The 3790T is beautiful. You don't even need to look at it. Its voice instructions are solid and you can navigate by them alone pretty much. With the TomTom XXL 540S if you try that, you'll instantaneously get lost. I mean that literally. You'll get lost immediately. If you listen to it, you'll end up turning all the time far before you need to. It instructs you to turn when the turn is 2 km ahead or 1.2 km ahead (depending on your speed).
I'd say go for the 3790T as it's the best unit available right now, and you're unlikely to find anything that performs better. I would however worry about the ads. You may be able to purchase the service to avoid ads. Not sure. (I was SHOCKED to hear that. It's bloody dangerous!)
Anyways, I hope that helps.
I should stop screwing around now and get back to work!