I just trashed my rear derailleur and need to get a new one. Upon looking for a new one, I realized my original was a medium cage. (SRAM X-7). All I could find locally were large cage derailleurs (X-7 or X-9). I understand that large cage derailleurs offer capacity for all the cog/chain ring combinations. I have always avoided going to the extreme combinations of small cog/small chain ring etc. and never had any issues with my medium cage derailleur i.e. I don't need the benefit a large cage would provide.

There are pros and cons to both the large and medium cage sizes, but what should I do - replace with a large cage or stick with a medium cage? What do most mountain bikes have? I am assuming long cage . . .

Any thoughts will be appreciated!