I have made all my raised beds out of treated wood but I always line them with old compost bags before filling with soil. The polythene takes away the contact with the soil which is what rots them out so quickly.
If you can afford it sleepers will last longer as they are so much bigger and wider? I have also recently built a raised bed using 3'x4' fence posts which if you shop around you can get fairly cheaply, these again are better than boards due to their thickness but cheaper than sleepers. With both these build them like bricks and the beds will look nice and neat also.
I do however think whatever you use once built paint the inside surface with preservative and line with polythene. this will protect the wood and also help to hold in some moisture (not that you'd need that with this years weather!!) One other thing worth considering is sitting the bed on simple gravel foundations which will drain and prevent the bottom boards from sitting in water or on damp earth?...
If you have a look at my blog I do have a couple of posts about building raised beds if it helps?