As Mike said, you're guy is a LF, so discount for that. How many years has he been pro? I'm assuming its been 4 years now, which is why you're getting the he's done growing comments, most guys don't improve much after year 4, some sooner.
Regarding the pitcher, what is your AS budget? How many seasons has he been pro? Was he drafted last year as a collegiate senior, or was he a high school draftee who's done growing?
I agree w/ Mike that your LF is the better player. Of course he'll be playing less than 100% a lot, or riding the pine a lot. The reliever could pick up a good number of innings out of the pen. unfortunetly, if he's your best reliever, than your bullpen is a little weak. You'll want your best relievers pitching a bulk of those innings, so in a good bullpen you won't see full value in his durability/stamina combo.
I'd only make the deal if your loaded w/ LF/1b types who are better than this guy, and you need relief help.
But for now, I assume he's cheap, and you may be able to get something better out of him later. You can pick up relievers w/ skill sets like that pretty easy (maybe on the durability/staming combos, but certainly the pitch variety/splits/control skills). Keep pimping him. If you keep him healthy, their might be a team fighting for a playoff spot who has an injury or needs an extra bat bench for the stretch run who would be willing to give up a better relief prospect.