Depends on the extension. I assumed more like $7M a year, which mean you'd be getting the cheaper and younger player. That's where I saw the add coming in.
Value-wise, I see RNH as a 60 pt center, since that's what we'd need him for, and that's what he's proven to be when he's not playing wing with an MVP center (which we sadly do not have in Winnipeg)
If he winds up with Ehlers' cap number, I'd do it as a one-for-one. 60 pt forward for a 60 pt forward. You get the younger guy, we get the more valuable position. That's about as even as it gets.