It's hard to compare across the two because it's not strictly apples to apples with the extra year of great value on Buchnevich contract. But that also means less likelihood of retention, whereas if the Pens move Guentzel they can probably swing retention to broaden his market on an expiring deal, which has value too, and appeal to a wider number of potential teams trying to maneuver tight cap situations.
As players, i think Buchnevich is the somewhat better player in a vacuum. But if you want a complementary piece to play with your #1C and just help do all the little things that elevate a line, with top line caliber scoring touch and productivity...you might just want Guentzel.
I think that's the real advantage with Guentzel. That you can rest pretty easy on the assumption that he's going to be adaptable and basically bring plug 'n play chemistry with whichever Top Center you want to pair him with. He's that sort of complementary "glue" player. Buchnevich is a bit more of a dominating player individually...which can be a good thing, or it can be a detriment if it doesn't end up being a natural chemistry fit with whoever you want him playing with. There's more "integration risk" with Buchnevich i think. As one of those wingers who plays a bit more like a traditional "center" in terms of controlling the puck and flow of play, where things run through him, compared to Guentzel who is better at doing the spadework but letting the play flow through that #1C.