My advice above all would be to try the pack on before you buy it. Everybody's back is a different shape and half the battle is finding one that suits you. Try to find a shop with a good selection and have a play around.
Above all never use anything in an ultra that you have not tested on a 20 mile training run, whether it is a pack, footwear, clothing or nutritional product.
My personal weapon of choice is the camelback octane 18, an I have friends who have got on well with packs from Inov8, Tera Novan and OMM.