I love the big betty!! the grip is the same as a maxxis super tacky as far as i can tell and ive had two pinches but they were both harsh landings that i dont think any tyre would have stopped , its a big tyre thou!!
I dont know if its duel ply but it has good puncture resistance and a lot of people use it if they are trying to save weight , ive had mine for about 3 months and wouldnt change it now.
the pure's great , didn't notice that much difference between it and the t-rex thou accept the weight of this bike with the different components is lots lighter which is cool!
for rocks i'd say st oyseth but clacton has street and rocks so depends what you want , they are only 4 miles apart anyway so if you get bored you can go clacton to.