Looking forward to the arrival of the 400 and the subsequent reviews. I'm in the market for this lens. If the weight is the only major difference with little to no difference in IQ, I'll snag version II. I don't mind the weight. I hand hold my 300 2.8 shooting basketball and volleyball all the time so a monopod on a 400 2.8 won't be a problem. $1,000 more for each pound of weight reduction is too much if that's the only difference.
I've just spent over a week using a 600 f4 II partially hand held. It is doable but really quite heavy.
I'm not particularily strong but I could get a minute or two handheld and it worked very well.
20% off the weight would help.
The 100-400 II felt lightweight when I switched over.
I find the 300 F2.8 II very hand holdable.