According to UAL, it doesn't matter.
They emphasize how the letter speaks of your character, whether it was a UAL pilot or someone outside.
And keep in mind, that 10 year FO may be your buddy, but not the company's buddy. Whereas the 2 year FO may have a better relationship with the company, etc. OR, where the "big 3" have 10k plus pilots on the list, 10 year guy can be just as anonymous as 2 year guy.