IMHO you want 3 test pieces with a variety of features, cut in material you supply back to back, the material needs to cross the range of what you cut and you stand there watch it and time it, material wants to be off the same sheet for each machine you test and realistic of what you cut, not some shit thats gone rusty - dragged out the bottom of the rack, this is not the place to be tight. Thats the only fair way to begin to compare things.
Mitsubishi is pretty well known world wide for there lasers, not sure i know any were with a LDV here in the uk, i do know more than a few of the Amada lasers back in the CO2 era had Mitz resonators as there beam sources.
Gotta say though some of the Fiber laser stuff i have had cut when fibers were new has been absolute shit, edges looked like the part had been chain drilled out of the sheet. Honestly could have been cut cleaner with air plasma. Thankfully they seam to have got a lot better since then. Thats why i strongly think you should be watching your cut files and samples come off the machine in person and no giving them the files ahead of time, you really want it to be a, walk on the demo floor and turn the machine on, cut the parts and turn the machine off to form a more honest picture.
Then don't let them baffle you with bullshit, because if they can not do it with your samples when your making the kinda purchase you are when buying one of these just how do you think its going to play out better when the machine hits your floor?