Well, nothing simple about lighting in this hobby!
The icecap ballasts are generally considered to be some of the best in the biz. They run cool, are small and light, and most say that electronic ballasts in general will help the life span of a bulb achieve its maximum. These ballasts also apparently save money in hydro costs although whether or not they save enough to make up the difference in purchase costs remains a little out of reach.
all that being said, do I run E Ballasts, nope! Compared to my pumps, the slight hum of my standard TAR ballast is not a problem, nor is the heat that it gives off, as a matter of fact it saves me a little in the winter. However, if storage of the ballast was to be somewhat enclosed, or if space was a major issue then I would eventually move to icecap.
I run 250's and am extremely happy, if you have the extra 400's already and I were in your shoes, I would probably go with 2x 250's and then maybe one of your 400's that way you can position certain corals under the lamp of choice.
Anyway, just food for thought. Good luck!
Rob