I decided against a Sport Tour simply because I want to go Cycle Camping (and will end up carrying most of my family's gear I suspect). The Tour is designed to carry lots of weight at the back and front. It's designed to handle safely and give heel clearance under these loads.
It's designed to take the full range of 26" tyres, from narrow slicks right up to wide off-roaders.
Basically it has the potential to be a Global Tourer but can be used as a commuter and a very comfy day-ride bike.
It's not super-quick but it is quicker than I imagined , the Sport Tour would obviously beat it in Sprints and through an obstacle course due to it's tighter geometry and lighter weight.
I think the Sport Tour would fulfil 90% of my requirements but lacks the Pack-Mule capability that I may need one day.
From your list of potential routes though (Sustrans, canal routes, towpaths, old railways, occasional off roading when it's necessary) I suspect you'd benefit from the larger range of tyre compatability of the Tour.
Don't worry about breaking it with 1.5" tyres, SJSC use Panaracer Hi-Roads on Tandems.
Like Stutho says though, you can't really make a mistake because you'll end up with a great bike either way.
PS. I bought my Tour with a long frame and Flat 'comfort' bars. I chnaged to Drops and this required a frame swap to a short frame to compensate for the difference in reach/position. Bear this in mind when selecting your bike, it's easier to adapt a short frame with drops to a flat-bar setup than it is to adapt a long frame with flats to a drop-bar setup.
Don't forget to post photos