After having two H3's with the I5, and now one with the V8, there is no way in hell I would ever go back to the I5.
The I5 is OK, but since you mentioned towing, I say go for the I5.
Your comment on giving the auto gas to go down hill is spot on!!!
The lower gear ratio on the manual? I'm not sure its worth that little bit.
Since you are going to be off roading this truck...ane more than likely adding 35's, I say go with the V8. You'll get the cast iron front diff, more power, torque for towing and moving larger tires around.
Go drive both and see what you think.