I've used Fender strings for years, not necessarily due to prefering them, but I had a friend in the trade who always gave me a few sets whenever we got together. Lost touch in recent years and have now used up all my supplies.
I recently purchased a couple of sets of rotosounds and have to say I'm not hugely impressed. I feel that they sound a bit too twangy when first fitted. sound ok once settled in and played for a couple of rehearsals and then very quickly sound dull, especially after a gig. This may be down to my sweaty mits, although I always take care to regularly wipe down the strings.
The fender strings didn't seem as harsh on the fingers and sounded zingier for longer.
Sorry, not a very technical reply, but I'm sure someone else will be along with a more detailed explanation.
Cheers, Gary.