I have both the 400 & 650. I love both. Right now my 24 yr old daughter is learning to ride it. I think she is having fun. But, when I take her to an empty parking lot in the industrial park, I feel like it is a toy over the much heavier 650. Actually, I feel more comfortable on it now that I have the heavier bike. I think the choice should be on how you are planning on riding it. Around town the 400 is great, even though I have had it on the Eastern Shore of Maryland as well as the mountains. It does the job.
But, my husband and I want to do long distance rides and I just thought the 650 would be more comfortable. Besides, he was getting a new bike, so I wanted a new bike. We both have MBS (multiple bike syndrome.)
We just went up in some hills of West Virginia on Sunday and I had the 650 and it handled great. But, a couple of weeks ago we were in the mountains of WV and I had the 400 and it was super. I actually think the 400 handles the tight turns better than the 650. You don't feel the constant shifting on the 400 that you do on the 650.