Hi Nick,
I currently have a Nuvi 660 and also a TomTom ONE and 910. I travel for a living and am in a different state just about every week. Here are my thoughts on your question, for what they are worth.
At this moment, the navteq maps seem to be better for the US. That's the main reason I have a Nuvi. I like the unit alot, but I prefer TomTom's interface and map screen. The Nuvi seems to have a much more accurate time of arrival, and it has a few features not found on the TomTom, such as travel guide, calculator, langauge translator, etc. I never use these features, but some people may.
I've been a longtime TomTom fan. I started with the Go 700 and used it for a few months. The maps weren't great, so I returned that and got the Nuvi. I got a really great price on the new ONE so picked that up recently as well, to compare how far TomTom has come. The maps are much better, and with the new maps being released in the next few weeks, I am very hopeful the TomTom will be my unit of choice.
Both are mac-compatible, but you have to go through a few hoops to update the Garmin's firmware using a mac. Garmin promises mac compatibility in the future, but right now they don't support it. There are ways around it, though, so that you are able to update firmware and even load voices and POI using a mac.
TomTom is fully mac compatible. The only real downside is that you can't control your device from TomTom HOME like fellow Windows users can. That feature is not supported on the mac yet.
I really like both Garmin and TomTom. I love TomTom's interface and the map display screen (gotta love that next turn arrow that's missing from Garmin!) but as of today, Garmin's maps are better. Again, I'm hoping this gets resolved with the new map updates.
Hope this helps you!