I've found that the kids really have a better time with the legos. It's an imagination thing.
Legos are also great because their limitations are consistent. There is no "better" way to have the Tuskin Raider hold his riffle. Articulation is no concern. There is no comparing one figures abilities to another.
Finally, in some cases, the ships are just better. They are to scale, they have amazing detail, and they can explode into a million pieces!
I'm a die hard ficure collector - but with a young child I've had much more fun with the legos - now if they would onyl go on sale more often...