I just don't know about the knife-first theory. I'm not passionate about it because the crime was just as gruesome and heinous either way. But I have trouble envisioning Arias taking a chance stabbing him with the possible physical altercation that might ensue, when a gun was at her disposal (assuming no stun gun for now).
I think her fine with the idea of raging on him with a knife, but ulitimately she was there for one reason, imo. She wanted him dead, and she at least wanted him knowing she was doing it just before it happened. Electing to start with the knife risked her desired outcome with a weapon available that was much more capable for the task, and that she had already staged events around to acquire.
I also have an easier time seeing Travis going to the mirror with the gun used first. If Jodi chose to start by stabbing away, I don't see where she would have stopped until he was incapacitated, especially if he managed to get to his feet and out of the shower stall. At that point I think she would have freaked a bit and the adrenaline would have kicked in even harder to make sure she put him down.
If she elected the gun first, I don't think she had a knife on her person. So if the former happened to jam she had to go get the latter. This allows him to stumble out of the shower, get to the mirror, and down the hallway during the ensuing fight - one in which she would clearly have the upperhand at that point. I then see the near-decapitating throat slash as the final FU.
jmho