The SQ5 is just an A4 Avant that's been jacked up a bit.... I wouldn't touch it with a barge pole, as it's the worst of everything for me.... but that's just my priorities, they're both VERY good cars in their own right*:
* I gladly advised my parents to buy an A6 2.0TDI saloon (2WD) for all their longer road trips and also a TFSI Q5 for their country ramblings (they live in very hilly countryside, do lots of hiking and mountain biking and most of the roads are impassible if there's anything more than a light frost)... those cars suited what they needed. They now do less of the long trips (retired, so they do more flying than driving), so they've ditched the A6.... those cars suited their priorities at the time.
My priorities:
I like a car that feels planted and fairly low on the ground, with ground clearance seldom (if ever) an issue.
I "need" quattro on the odd occaision, but all it ever sees is the odd icy hill / country track and / a few wet / muddy fields... .a decent set of winter tyres would probably suffice, but it's nice to know that it is there the whole time.
I really DO need a big back seat (three child seats) and a big boot (three children's, double buggy, bikes, scooters, etc).
You seem fairly set on the BiTDI, and it's a great engine, so if diesel fits your usage profile, then that's the best big diesel out there: but what little mileage I do consists of a fair number or medium to short trips and the odd really long one, so I would just damage the diesel engine with those short trips and I wouldn't really make use of the extra economy, plus I like the idea of being able to eventually do a cheapish pulley+remap and push it WAY over 400bhp.
The A6 Avant (or A7 as a backup) are the only real options I'd go for with the A6 Allroad/Q7 as outside runners (same size car, but just jacked up: more practical offroad, less practical on road).
Work out (or tell us) your main priorities and then it will help people advise which car is best for you.