The Garmin traffic facility may be free - but my understanding is that’s because it only uses the free-to-air RDS-TMC system. Better than no traffic info at all, for sure - but sadly it only really has info for the trunk roads and motorways (and not very detailed info at that). So, while it’s OK at telling you about a big delay on the major route you are currently following, if you follow its suggested alternative route you stand a 50% chance of being plonked straight into an even longer queue on the B-roads (because everyone else is going that way). Some TomToms (such as the old Go7000 Truck) use the same RDS-TMC system, but the newer units (including the 5150) use TT’s own subscription-based “HD Traffic” which is head and shoulders (and the rest) better, in my view. Well worth the annual subscription, I’d say. Weather and Google searches are good, but you can get the same (and better) from a smartphone.
I’ve used Garmin Diezl (not sure which exact model it was) as well as numerous TomToms. The Garmin unit does have some better features - such as the ability to optimise a multi-drop run with a couple of taps. In fact TomTom seem to have seriously dropped the ball with the multi-drop features - Their satnavs always used to have “Itinerary Planning” that enabled you to easily punch in your day’s drops then tick them off and move onto the next with a couple of screen taps. That feature has been lost, and their “Travel Via” fudge doesn’t really cut the mustard.
Both claim to give truck-friendly routes. In my experience, the TomTom seems much keener to keep you on A-roads as much as possible, even if it adds a few miles to the journey. This is No Bad Thing in my book. Mapping data comes from the same original sources. I don’t think either maker has “better” maps than the other - you will still come across “missing” height and weight restrictions whichever you choose.
If you frequently change between a couple of different trucks or trailers, you’ll like the Garmin better - you can save custom configurations and switch between them quickly. With the TomTom you have to go through all the vehicle parameters in turn (height, weight, length, speed etc) changing them as required - no facility to store your favourite configurations.
Away from the truck, both can be used in the car without the height/weight etc restrictions and using “normal” routing. The Garmin comes with its own route planning software that runs on a PC and allows you to pre-plan a detailed route. TomTom doesn’t have anything quite as sophisticated - although 3rd party programs such as Tyre (tyretotravel.com) are arguably even better anyway.
I’ve been very happy with TomTom products over the years (at the last count I’ve owned at least 7 and we still have four of them in the household now) and am very happy indeed with my current 5150 (and the older Rider 2 that I use on the bike). Historically, I think Garmin hardware generally had the edge, but the TomTom software was slicker and easier to use. Those differences have eroded over the years (and TomTom units have in many ways “dumbed down” the features offered), untill now I don’t think there’s a great deal to choose between them.
Oh - and TomTom tech support, should you need it, is pretty good once you’ve got past the initial script-driven first-line support droids and started communicating with someone who has a clue.