I'm a USU alum.
Mormonism:
The Mormon presence at USU has become stronger over the last 10 years or so. Believe it or not, in the late 80s USU was ranked among the some of the hardest partying schools in the country.
There are plent of people at USU that are not Mormon, but the religion issue seems to hover over most social interactions on campus--even among circles that aren't religious. You'll also find, especially in the engineering department, that most of your classmates are either married mormon males that have returned from two year missions or foreign students. Both groups can be cool, but the number of "regular" dudes is going to be small.
A lot of people will warn you off about the mormons. When all is said and done, they are decent folks. The worst thing you'll get from *some* of them is some phony buddying up in an attempt to convert you. Mormon girls may date you, but you won't make it past being friends. Also some of the non-mormons are little bitter and like to act out a little in being rebels.
USU:
It's a large public institution. You'll be a number, but you'll get a solid education. The College of Engineering is probably the most well-funded and most prestigous of USU's colleges. You can't go wrong with an engineering degree from USU.
The campus is beautiful. There are tons of intramural sports, outdoor clubs, and recreation classes. I took a free course in rock climbing and fly fishing while I was there.
Logan:
Logan/Cache Valley is one of the most beautiful places in the world. If you like doing stuff outside it is an incredible place to live. You name it, they've got it within 1 hr.
There isn't much of a night life and for some that is boring. There is virtually no crime in Logan. I own rental house a few blocks from USU and for over two years while I renovated it I kept tools scattered all over the place. The only thing I ever had "stolen" were the pears off a tree in the yard by some hungry students.
The wages in Logan are low because there are lots of broke students. But the cost of living is also low, so it's basically a wash.
My Advice:
If you are a real outdoor nut and/or really want to do engineering, USU is going to be a good fit despite the diminished social scene. But good fit and right fit are not the same. IMO, aside from getting a good education college is also about having fun and getting chicks. I'm assuming you are in your 20s. If so, there is no reason you can't have you cake and eat it too by going to another school. My advice would be to fly into SLC and take a road trip north to all the schools.
Best of luck.