If you can't do better than an average of 70 on the freeway you have something wrong with your engine. I can set my cruise on 80 and don't have a problem and I can zip up and down these WV mountains with no problems carrying 300 lbs. in tools and my 200 lbs. with my 01, 2.7 auto entended cab 4x4 and 31" tires.
Off road there are places where I wish I had a little more power but placeing it in 4 wheel takes care of that.
Even though I love my 2.7 I lived in Golden for a while and spent alot of time in the mountains and much time on Wolf Creek pass and up on the divide. I do believe I would go with the 6 banger and an auto. Way to much shifting for a stick

Good luck with your new Toy! Mike