Tough choice between the two! The Outback is bigger, but the Forester is higher, with 20 mm more ground clearance. Either way, they are both solid cars with good safety ratings.
I've also heard that the diesel is iffy (plus they don't sell them anymore, because???) They did do an Outback diesel with a CVT (auto) 'box until a couple of years ago (stopped around 67 or 18 reg), but beyond that you'll need a petrol to get the CVT. Since then most (possibly all?) have CVT as their Eyesight safety system only works with that gearbox.
I've not seen wading depths listed but the air intake is on the bonnet slam panel, so quite high. (Whether it will float before then, I'm not sure!) I think the Outback has a greater towing weight, too.
MPG sounds about right for the petrol. My wife's XV (2.0 petrol) gets low 30s pottering about but high 40s on a run. My Levorg (1.6T) gets high 20s and mid 40s respectively.
I think either would suit your wife. Maybe the Outback is better for chucking a bike in the boot and towing, but the Forester has the ride-height. Your best bet is to take a trip to Stan Palmer's in Rosehill and have a look / test drive them.
(Either way, make sure the one you get has Apple CarPlay / Android Auto, as the standard sat-nav is oblivious to traffic and expensive to update.)
Good luck and let us know how you get on.