I look at lab tests and also try to get some experience playing with a rig on the air before I commit to one. I have owned the 756Pro and TS-2000 for over a year and sold them to buy the Mark V, which I have not owned very long at all. I also had an Omni V for a short while.
I guess my opinion is that while you can work DXCC and even get on the honor roll with the 756Pro, the TS-2000 or the Mark V, that the Mark V is the best for HF dx (if that is the only thing you want to do). On a budget, a used Omni V makes a lot of sense. And no matter what - its operator skill, your antenna system and then the rig as far as DXCC - ability is concerned.
I found that the 756Pro and the TS-2000 receivers were (on the air) very similar in their ability to receive weak signal DX. The TS-2000 is a bit noisier and has some nasty birdies. I went from the 756Pro to the TS-2000 because I wanted work satellites when HF conditions were in the dumper. That is still a good choice if you only want one rig to do it all. And it was great for the VHF contests too. Like a swiss army knife - it works but some blades / tools in the knife work better than others.
I had the 756Pro and never looked at the fancy LCD screen - I was too busy listening and trying to work weak signal DX to care what their signal looked like. I admit - in the store the screen surely was enticing. Others may actually use and love that feature - I did not. It didn't do anything to further my DXCC goal. If the satellite bug didn't bite me, and I wanted to choose between the TS-2000 and 756Pro for HF DX-ing, I would have kept the 756Pro over the TS-2000.
I also had the priviledge to play with an Omni V for quite a few months. That was a super CW rig for DX-ing, and I think it is the best deal for used market CW dx-ing rigs. It wasn't that great sounding on SSB. But I work mostly CW unless a new one is SSB only - which happens quite a bit. It is a 100W rig and there are some well published mods and firmware upgrades to make it a super duper rig.
I like all of these rigs - its just that after a few years of being back on the air I have noticed that I spend all of my ham time chasing DX, and that for me, the Mark V is the best rig I've come across for doing this one ham activity.
After I reach honor roll and 5BDXCC QRO, I will try it QRP - probably with a K2.
And remember, these are just my opinions and my experience - your mileage will most likely vary.