We might get more for Petry at the deadline (two playoff runs on a cheap contract if MON holds back 2.75M for next year) because, at 33 1/2 when his next contract starts, there might not be that many willing to pay him the amount he'll seek for the term he'll want in a sign and trade scenario.
Tatar, on the other hand, will be as old as Pacioretty was when he was traded in a sign and deal scenario with Las Vegas. Tatar will have more value on a 4 or 5 year extension, like what was given to Pacioretty.
Wait with Tatar and see how costly it would be to re-sign him. Move ahead with Petry, IMO, unless you can get Petry to accept an extension of no more than three years at no more than 6M. Not likely.
I believe that you're better off trading a more pouty Domi than Tatar, if the return is good. I know it doesn't sit with the getting younger motif but, Tatar can still contribute to a Cup window, IMO, and that leaves enough time to draft and develop a replacement or two for Domi and Tatar, on top of what we might get for Domi.
IMO, Domi should net us a solid D prospect from a team like EDM.