I find most vehicle rental companies are basically the same, I've used Fox a few times because they are often cheaper than the competition (at least in Vegas and LA) and I haven't had any issues with them (unless you count long-ish wait times, but my experience has been that way with most vehicle rental companies unless it's not a busy location). Check with the review sites (TripAdvisor and those kinds of sites) to see how the company compares to other rentals where you are headed.
I find the main issue with all the car rental companies is the insurance. Should you get the rental company insurance? Should you use your credit card insurance? Should you use your existing vehicle insurance? Should you use the insurance that you can get through the company you rented the car through (expedia/momondo/travelocity/orbitz etc). It really depends on your situation and where you are going. I've used my credit card insurance sometimes, I've used the rental companies insurance other times, and once I used the Expedia insurance.
Weigh the pros and cons of the extra insurances. My last trip to Iceland we weighed heavily on whether or not to get the 'sand & ash' coverage because we were going to be driving through the south near where they (very seldomly) have wind storms that can cause the sand to blow against the car and strip all the paint. We decided against the extra insurance and went with the credit card insurance. But check what your credit card insurance actually covers - and some of them require you to waive all other insurances (even if it's a basic one that's included with the rental) in order for theirs to be valid.