I agree and disagree on some of the points you made. But I'll touch on their common opponent Lucian Bute. Both fights imo were pretty close fights but I think DeGale struggled more with Bute than Jack did. That points to who can likely stay more disciplined in the ring. My money is on Jack, and if the going gets tough and both want to slug it out, I think Jack has the slightly better Technique and his punches are more short and compact, while DeGale's are a little wide and loopier. And shorter shots get to their target first. It's going to be a very competitive fight.
DeGale by MD for me, I think DeGale wins the early rounds easily enough and then eases up and barely manages to eke out the decision.
DeGale, UD or TKO past round 6. If he want to show himself to the world and start building a name in the division he'll take the offensive (in an intelligent and sharp way), once that happens Jack will stay worried and will lose round by round as he tries to get a solid counter punch with his back on the ropes.
I don't think it will be an easy fight for DeGale. But I do agree DeGale is on another level in terms of skill and versatility. The problem with DeGale is that he switches off during fights. I'm sure he will dominate early but at some point he'll just lose focus and he'll let Jack back into the fight and then pick it back up at some point to squeak a close decision.