I have both and I LOVE the xxx-4 and it is the most fun I have. However, be prepared for some spare parts unless you are a very good driver. The xxx-4 Graphite edition is a great value for all of the race parts you get, you really don't have to do any hop ups on it. Unfortunately, the 4wd class usually isn't a large class. Check at your track and see how many are showing up on a weekly basis.
The truck is easier to drive, more forgiving and more durable. I just can't believe that the 4wd class isn't the largest class. We are always able to race, but around here the 2w mod and truck classes are huge.
I am sure you will be happy with both, but I haven't raced my truck since I but the 4wd.