just to expand on what rocky has said..
a) They weigh about 110 pounds when full. I find that (with two people) you can lift it up or move it with just a little effort.
b) Yes, they are more fragile. Although more fragile than a carboy, demijohns are still surprisingly tough. at one time I has some 45 of them (yes 45 of them) and have only has two or three break over a 12 year period.
The best part is that most demijohns come in a plastic bucket (avoid the wicker wrapped ones). When a DJ breaks, the bucket catches most of the wine. I find that 2 carbays can be saved in this case.
c) I find that "splitting a batch" into two carboys, when one carboy has a problem, the other one will too. I have never really had a case where the same wine, from the same batch of grapes, treated the same way, gave drastically different results. That is my own experience, but it is not to say that this is not possible.
I remember the same concerns when I purchased my first DJ. I remember the fear of "all my eggs in one basket". I have found, when you reach a certian point in the growth of your winery, going to DJs is the best choice.
By the look of things, you have reached that point!