Hi Jo
In the Monty Don allotment special, they showed a row of very healthy looking raspberries mulched to about 6-8" (that's deep!) - can't remember what with - wood chippings I think...
I'd be inclined to use compost if you can (add plenty of straw when making the compost to it to bulk it out, maybe?) The mulch is to retain moisture and supress weeds rather than anything else, but the compost will improve the soil too - double bubble! :D
