I have used both over the 30+ years I have been quilting. They both wear about the same. By the time I am done with one it is unusable. Both of the are equal in their self healing abilities. Fiskar is a bit heavier than Olfa. Pricewise the better deal IMHO is Olfa. Olaf's has a better guarantee if you ever need to return a defective product. That applies to their rulers too. Fiskar's lines on the mat will wear quicker that the Olfa. Both of them are good for use on the opposite sides too.
I also have the 54" Olfa and love it! When it comes to replacing my boards it is always price dependent anymore . I also have a small Ofla that I use when going out with my machine. Easy to carry. The smaller Fiskar is not as strong,.