I was in Estes Park last weekend. It's only an 80 mile drive for me.
I25 has some rough patches and has a lot of traffic. I76 is a good road and a little less traveled. I would just pick whichever is shorter for you.
Like warsw said, 34 is twisty, but not bad. The steep parts are mostly speed limit 45 anyway. I have a 34 ft 5er behind a 2011 Duramax and did not have any trouble. If you have a gasser, you may be flooring it in places. I find the return trip a little more challenging. Lots of downhill with twisty. Your brakes may heat up.
The elk wandered though our camp ground a few times. Have a fun trip.