I've seen TomTom and it works very well - no wonder it's so popular.
However, although the Audi RNS-E unit is more expensive, and can be tricky to fit and remove if you're not technically-minded, financially it's not all bad. You could resell the RNS-E separately before you sell the car to recoup most of the original outlay. And once in it looks great, plus you've got Bluetooth and TV upgrade options available to add to it.
The reason I'm not a fan of aftermarket sat-nav like TomTom is that one, it doesn't integrate properly with the car and looks a bit daft hanging from the windscreen with suckers, and second, there's been a massive increase in theft - the pondlife see the car adapters and holders, and break-in to try and nab the lot, even if the owner has taken it away with them.