JvR has size on Kessel, but he's not very physical. He likes to mix it up in front of the opposing net, does a very good Holmstrom impression which is a huge plus, but other than that he's not very physical. Compared to other big forwards, he's way more of a Spezza/Malkin type than a Getzlaf/Simmonds type. Also, he doesn't offer a tonne defensively over Kessel either, he seems like he COULD be a good defensive player, but really doesn't put in much effort. I'm not sure if it's Kessel, Lupul and the like rubbing off on him, but he's pretty passive defensively, not a guy who's really hustling to recover the puck.
So, while JvR does have size on Kessel, it doesn't translate to him being much more physical, or much better defensively. On the other hand, Kessel has a LOT of offensive talent and effectiveness on JvR, which makes a big difference.
For pure on-ice impact, I take Kessel over JvR by a pretty wide margin. Where it gets trickier, though, is when you consider:
- JvR has a $4.25 mil cap hit, Kessel has an $8 mil cap hit
- JvR is 2 years younger (25 vs. 27), AND seems to be a later peaker. Kessel is probably at his peak right now, while JvR seems to be still improving
- Kessel just gives off a lazy, cynical, "not team player" vibe. JvR seems like your pretty standard, cheerful hockey personality, Kessel always feels a bit "too good for the team" - openly stating that he doesn't practice in the offseason, acknowledging that there's basically a separate set of rules for him vs. everyone else on the team, regularly complaining about what a "gong show" Toronto media is, etc. I'd guess that Kessel is a bit of a locker-room cancer, while JvR seems like he'd be a solid locker-room guy
If it was a "lose one of these guys today" situation, I might just have to keep JvR, for the 3 reasons above (half the cap hit, younger/still improving, seems to be a better personality to build around), but I do think Kessel is quite a lot more effective on the ice today.