Shelter 501 MKII vs. Sumiko Celebration on a P25?

Any opinions on those two carts would be greatly appreciated.

My opinion is worth what you're paying for it. I like the 501 better than the 901, so a good choice. The Celebration is one of my favorites. I doubt it enjoys the qualities that some golden ears seek but it's one of the most enjoyable cartridges I've ever listened to and in its price range is a steal. FWIW, I recently went with a ZYX. There is a lot of new technology going on with this lineup. If you can imagine the best qualities of the Shelter line being retained with a smoothness akin to silk then this is very much worth your consideration. Two things about buying a ZYX. First, the importer, Sorasound, is worth doing business with. I cannot speak highly enough of the way he is doing business getting this line off the ground here in the US. Second, just arrange a phone call to him and do not go to the webiste link. IMO, it's more confusing than helpful. Also, there are members here that will help you a great deal with matching of the rest of your system to the right cartridge. The current pricing structure is very much in your favor too. Good luck but I doubt you'll need it. Both the 501 and Celebration are wonderful even if you don't consider the ZYX line.
Thank you very much for the excellent reply. Much appreciated!!!

I have been looking at the ZYX Fuji F100 FS... Something else to consider!

What do you think of the Benz Ruby 2H?

Sorry, but I haven't listened to the Benz line for quite some time. My memories of ten years ago were that it was a very good product. Still, my personal tastes have changed toward more detail/articulation and if the Benz house sound is the same now as before I stand by my earlier recommendations. Benz is a very respected product line that has done well by us audiophiles.