I actually started with the Ibex 144 and felt it was too long for me, not very easy to maneuver in the air so I went back down to the 139 and love it. I'm 6'3" 210lbs. I have never rode the witness, but have to agree with Scholedrider & Andrew, keep you witness until you find that new board you like. The Ibex has some of the best pop that I have ever experienced on a board and holds it's edge really well.
All comes down to personal preference and what you feel comfortable on. My best suggestion is take your witness with you and ride back to back sessions to determine if there is enough of a noticeable difference that you like about the new board vs your current board. Try different lengths as well, do basic cuts, slides, jumps, etc, and have fun with it.