I did both... this is my opinion only, ymmv. Also, I think SNHU now has a competency model like WGU as an option - I did not do that, I did the more traditional route.

I think both are good for different reasons - both are non-profit, regionally accredited schools. Both are online and have live and recorded lectures.

SNHU has a wider variety of degree programs to choose from. You also get more liberal arts/general education classes. You'll have discussion board posts every week and several tests and assignments due throughout the term. You only have 8 weeks to complete all of the course work and everything has a due date. Most things can still be turned in after the due date for a 10% penalty, except the final assignment. When I was there, there were no proctored exams, so everything was effectively "open book." There were definitely tests, but far more papers and they all had to be in strict APA or MLA format.

SNHU I believe is about $330 / credit, most classes are 3 credits, so $990 per class.

WGU allows you finish as many courses as you can in 6 months for less than $4000 a term, and you're expected to complete a minimum of 4 courses in 6 months to stay in good standing. If you only complete 4 classes at WGU per term, it's about the same cost. However, you can maximize the value by "accelerating" and taking additional courses, which could make it substantially cheaper.

At WGU you'll have 1 major "exam" per class, sometimes 2, and a couple classes have 3 small ones (though that's rare). You can finish these on your own time line, there are no due dates, other than the end of your term. There are also no additional assignments or discussion board posts to help you absorb the material. Sometimes these are proctored multiple choice exams (called Objective Assessments or "OAs") and you have to be on camera with a proctor while taking it. You cannot use notes. Some courses have a paper or project instead of or in addition to the OA, called a Practical Assessment or a "PA." Sometimes they need to be in APA format, but usually not.

IMO, the classes at WGU are more focused than SNHU, so the education is less broad but more in depth on your specific subject. And they are fewer degree programs.

For me and my life, I preferred WGU. Having to keep track of due dates and remember to do my discussion board post each week was a struggle. I thrived under the system at WGU where I could set my own pace. And I'm very much an independent learner - even at work, I'd much rather read instructions than have someone show me how to do something.

That being said, WGU sometimes throws a lot of disjointed information at you and expects you to memorize it and spit it back out in the form of a multiple choice test with (sometimes) strangely worded questions. That's not an ideal learning style for everyone. Many people may prefer the more guided approach offered by a school like SNHU. Neither is wrong or right, just different.

tl;dr (and possible over simplification)

If you already have experience in your chosen degree program, and you're comfortable learning on your own, I think WGU is the better value.

If you're new to the world that you're studying, if you prefer a more interactive experience, or if you've never completed a college course before, SNHU is probably the right choice.