I say Jace. While Jack Stryker is probably his most oft used and lets say, "well developed" personality, Jack is pretty much a "tough guy" persona Jace resorts to whenever he cannot deal with some stress, or thinks that the tough guy marine act will get what JACE needs or wants, not necessarily what Jack needs or wants. Remember when Jace mentions his trial for his weed DUI? He pulls out Jack Stryker during his plea bargain, asking, "Is that really the best offer you can make to a Marine!?" to the Judge in hopes of a more favorable outcome; Jack didn't suddenly take control and start demanding things, Jace
decided to use him.
The best example of this I think is during the Muslim Ghost incident while Team Gamerfood was calling Jace. For nearly all of the calls, Jace is himself in dealing with Demon Lord FlyAwayN0w and the Muslim Ghost, but the calls aren't going his way at all - he's taking demands, nearly getting into fights with black people in Starbucks that the ghost turned gay, he can't find where FAN is in order to make him stop reviewing his books, in short things are not going well for
Jace, so he instead tries Jack Stryker when his phone rings next time.
But as the call shows, his tough guy persona is torn down just as quickly as its put up from the right trigger - Jack is "in control" all of 30 seconds, before somebody threatens to kill a made up character in his mind and drops a heavy book near a microphone, and everything twists quickly back to Jace (who screams like a howler monkey, seriously I've never heard such inhuman noises come from a person).
I can't say much for this new Wolfsona but as far as I can tell that's just how Jace identifies himself - its not really anything like Jack Stryker, which is a separate entity. Jace admits "he likes to be Jack" when things are boring, so he stops taking his meds so he can be Jack Stryker. This new Wolfsona
just is Jace - he doesn't "decide" to be it, and in fact rather angrily reacts when people call into his stream and ask about his "decision" to have a wolf soul, responding with "I didn't choose to have one!"
All that being said, I'm kind of curious what Jace's response would be if he was questioned about the relationship between his wolf soul and Jack Stryker, ("Does Jack have a wolf soul" and such questions).