Hello,
At the end of the day if the purpose is for rentals then I would think demand (present and predictable future) and upfront cost would be the driving force.
As for my opinion the VTX system kills the VDOCS as it should. As stated above its ten years of advancements in R&D, DSP, and good old design experiance. Not to mention a major part of the LAcoustics team is now at JBL.
The real comparison is between K1 and VTX. I have not personally mixed on the K1 but I hear great things. It is my opinion that at that level of quality its hard to say a bad thing about either. If you cant make it sound sound great on either PA system then.... it's not the gears fault.
As for amp purchase, both companies will lock you into amp packages. JBL offers their "VRack" it can either have iTech 12000 HD's or iTech3500's. The rack loaded with 12000's is going to give you the most out of the system but it is a much higher initial investment.
Cheers and happy purchasing!
http://flagsystemsinc.com/