I switch favorites myself, but more often then not it's fantasy that holds my interest... even with edition shifts.
I guess a lot of it might be the lack of an MEQ type situation, where half the armies seem to be the same army. Add in the dislike I have for tanks and well... I guess the aesthetic is a huge appeal.
Personally I think both have a very well designed story. And despite both being heavily influenced by other materiels I think both have a rather unique story line at this point.