The fine cut deck has front baffles installed, and is a little taller. And then... you can add a deeper internal baffle, and block off to it. (The mulch kit) The standard deck doesn't have a mulch kit. (but gives better ground clearance)
I went thought the same thing when I bought my tractor. So... what I discovered is... the "Fine Cut" doesn't refer to the actual cut quality. It has to do with the clippings that are discharged. So, your dealer is right... the "Cut Quality" is good with either deck.
After looking at both decks... and knowing that in the fall, I needed a deck that could suck up A LOT of leaves... I started to think that the fine cut deck may not be able to swallow that. So, I decide on the standard deck. After using it for a full season last year... I'm glad I did. Here's why.
1) The speed and velocity of either deck is good, and I don't bag my normal clippings. Even with the standard deck, I don't see any clippings in the yard. (assuming I haven't let it go until it was knee deep)
2) The gear box on the standard deck is a little more robust. (don't know, just my observation)
3) better ground clearance when just driving around.
4) The actual "Mulch Kit" for the fine deck is an extra expense.
5) Since the front of the standard deck doesn't have any baffles... it's able to suck up a large amount of leaves, chew them up... and spit them out during the fall. (I converted an old Ingersoll bagger, and it never clogged on me)
That's my story, and I'm sticking to it.