I also fly Pa28's and 2160's out of Sherburn, having converted to the Robin recently.
The 2160 is indeed very nice to fly, although it's a bit more of a handful than the slower and more stable Pipers and Cessnas. Looking back to my basic training, I had my hands full handling the Pa28 in those first few hours and the Robin would have been even harder work. Depends what kind of person you are - if you like the 'in at the deep end' approach go for the Robin. It's nice, and as you say, £5/hr makes little difference.
If I was training again, I'd split my training between both types, with some C150 work thrown in too for good measure. That way I'd come out of it a more rounded pilot. Do the first stage of the course (to solo consolidation) in one type then experiment a bit with others.
As for availability, yes, the Pa28's are more usually used for training, but some do train on the Robins. Since converting I have found that Robins are easier to book at short notice because, although there are only two, less people use them. If you plan well ahead you should have no problems getting the bookings you require.
PM me and I'll happily chat in more detail if you want.