[Perdu]

Personally, I would not switch a SubC for a 2500. The 8500 is a superb movement but there are few watchmakers who know how to service it, whereas just about any watchmaker can service a 3135. Movement wise I would put them in this order

8500
3135
2500

but these movements are all chronometer grade and properly regulated are capable of the same excellent time keeping.

There are many other factors besides movement you need to take into consideration, like which watch do you like on your wrist and resale values if you think you will ever flip. Let us know what you decide.