I have a similar setup to what you're looking at in the toyota/trailer, go with the 450 and small enclosed. Fuel will be slightly higher/mile but if you use the full capacity of the truck it will be cheaper and you will be able to do work more efficently day to day. Oil changes will be cheaper as you don't change it as often. 10K miles $100, or $40 for 3K miles.
Knowledge is knowing a tomato is a fruit, wisdom is not using it in a fruit salad.