Depends what you want to do with it really: Plug in and go then get a TomTom, Ultimate control and flexibility get a Garmin.
I'll expand on the above statement:
It used to be the case that the TomTom had the better user interface, however Garmin have played catch-up. I have two Garmin units, a Zumo 660 and a Nuvi 765 plus a TomTom Go 730. There are positives and negatives to both in respect of the UI. Whist the TT Go 730 isn't a Rider there are many family similarities.
The direct routing on both is comparable, though I haven't had the TomTom very long. Those that say that their Garmin does some very odd things need to look at what avoidances are set. The Zumo 660 comes out of the box set-up with a few and the selection of driving profiles adds to this confusion. Trying two different standard routes (just A to B) between the brands (using the models I own) and the Garmin, for me, provides a route that I would be more likely to take. The TomTom seems to favour back lanes marginally more, this is with both on the fastest route setting. Obviously the TT Rider has the Winding Roads feature specifically for bikers. In principal it sounds good though you are still at the mercy of the TomTom written algorithm that dictates what route you'll take. For just heading off into the unknown I would mark that as a point in the TomTom Rider's favour. I would be interested to hear how effective this is.
Personally I want to be able to use my sat nav in two ways: 1/ just get me from where I am now to where I need to be with least fuss and as quickly as possible, 2/ let me plot a detailed route on my PC, maximising my ability to research where I'm going and where I'll be visiting. Then transfer this to the GPS and it follow this route. Here, to my mind the Garmin scores as it has it's own dedicated route plotting software in the form of Mapsource. Yes, there are third party solutions for TomTom but these rely on Google Maps API for mapping and route preview, your TomTom uses different mapping and different algorithms to interpret the parts of the route between any way-points. Usually this isn't an issue, especially if you add sufficient via-points.
TYRE and
ITN Converter are the main contenders for TomTom Google based route plotting and these provide a somewhat more simplified interface than Basecamp but at the same time capable of less. Hence Basecamp does take some persistence to master, once you've done so though it will be a more powerful tool.
Basecamp has a degree of Google Earth integration which can be very useful for research purposes. ITN Converter also has Google Street View. Both TYRE and ITN Converter can also be used for Garmin route plotting. I use the latter quite regularly as it has the ability to display Michelin maps (useful for those green edged roads).
If you miss a via-point with a pre defined route on a TomTom it will continue to insist on you turning around and revisiting that point unless you stop and manually mark it as visited. Garmins will try and prompt you a few times and then assume that what you're doing is intentional so continue with the route from where you are currently. Again this is where Garmin and Basecamp can score again as there are several types of points that can be used along your route; aside from start and end there's way-points (an actual place to visit such as a hotel, restaurant, city centre or a user created one), routing node (points that you've added to the route to influence it's course to follow the roads you want to travel) and invisible nodes (points that Basecamp adds unseen along your defined route). The first will, quite rightly, not be ignored. For the 2nd a few attempts will be made to try an get you to route though, or pass in close proximity to, them. The last remain unseen and be ignored if not visited.
The Garmin can take open source third party mapping data in the form of
OpenStreetMaps, this is detailed and up to date. Lacks a few bells and whistles of the Garmin data but for core navigation functionality I must say that it's pretty good. Ideal if you're somewhere outside of the area covered by your mapping, i.e. on holiday. Coverage across the globe is pretty good as well, covering some areas that Garmin, and TomTom, don't.
Whilst it's possible, with some compromise in terms of weatherproofing etc, to use cheaper car sat navs from both manufacturers on a bike, be aware that not all will do routes that you can define more than the start (where you are currently) and the end point. For recreational bike use the ability to plot a convoluted route, usually on your PC, is IMHO essential.
As they say, the decision is yours...
