If you are mainly a pellet shooter, I would say, the 600 mm is better. Why: Because you can carry it in one hand along your side, without fear of the barrel bumping on the ground. And if you like a gun, capable of using in a truck, moving around from either passenger side, or backseat, out your driver window, shorter is better. But if you paid extra for a 700mm, I would keep the 600mm, and ask for a refund of for the inbetween. The most important thing is that the gun shoot good, and you are happy with how it performs
You raise some very good points, I do like not having to worry about bumping the ground with the barrel.
The velocity to which you can push a pellet while remaining stable depends on quite a few factors, one important one being efficiency, meaning if you're at roughly 1.3 fpe/ci or better, efficiency shouldn't be causing issues, but under that, there is an increased chance of turbulence effecting the pellet as its leaving the muzzle...The reason to that is simple and should be elementary with tuning in a particular pellet for any gun...
So while triggertreat may not be able to shoot an mkll at 920 fps without his groupings opening up, many others may very well be able to, even with their impact mk ll... (there are those that already do...)
I have the Ronin on my 700, and it doesn't bump the ground.
How tall are you?
That barrel looks like a 600mm barrel.
The first picture is a 600mm barrel, the seccond 700mm. Both guns using the same FX stock moderator. I know it is a bad picture, but tried to take both on same angle and position. For reference I also put a stock FX moderator on the floor, which is 12.5 cm long. I am 178 cm tall, and believe I have normal length arms