I have had a Garmin 660 and now have a TomTom Rider 450.
I found the 660 easier to plan routes on the device, also more intuitive in general, also would revise the route if you decided to do a detour or start mid way on a saved route. 660 can play music which is nice on motorway.
The TomTom does not like you varying from a plotted route, it ends up sending you on a crazy detour to try get back on track, did it to me twice in Scotland. The TomTom PC based planning tool is good, can be used when the device is not connected, then synched later. The TomTom winding routes function is interseting but sends you into every long layby and through the centre of villages instead of sensible route around, means that the distance to turn is not accurate if you are say staying on the A82 then turning onto the A86 after approx 50 miles, it says 2 miles to turn (which is a layby) then 3 miles to turn (another layby)... I find Copilot on my phone when set to avoid motorways, avoid major routes, favour minor routes gives same routing without laybys and does not do the wild goose chase if I try to rejoin the route part way through. Trouble is my phone will not respond to gloved hands......