I went with 42" forks by Quick Attach. They are rated at around 4000 lbs and are very well made. It is not that easy to slide the forks when you need to change widths, but I don't know how they compare to others. I went with 42" because one of my main uses is moving boulders. If you intend to move a variety of pallets, be aware that some (most?) pallets require 48" forks to reach the far rail. (I would probably buy the same brand but 48" if buying again.) The loader on the MX5100 will lift about a ton off the ground, so going with forks in the 3,000 to 4,000 lb range is not over-kill.
If you have the opportunity to try any forks on an MX5100 at a dealer, take note of visibility through the back of the forks. On a tractor FEL, it is more difficult to see your forks than on a skid steer or fork lift.
Overall, I've found pallet forks to be one of the most useful attachments. I use them almost as often as my bucket.