Between those two choices, I would pick up an Empire Army. They have lots of different units, and have numerous builds to fit any play style. They have really cool war-machines/ unique units like Steam tanks, decent Cavalry units, as well as some nice foot sloggin units like Great swords, and Halbardiers. The empire has decent shooting units like pistoliers too.
Most of the Empire sculpts are awesome. Painted Flagellants on the battlefield look nice. Not to mention the Empire has conscripts from other "kingdoms" making it easy to have themed armies of a province.
I really like the history/Lore of the Empire (except the part about helping the ugly Dwarf race beat down Greenskins).
Rumors have been pointing to the Empire getting the next Army Book. Maybe as early as June (likely 3 or 4 quarter knowing GW). Along with those rumors are new or "reintroduced" units from older Army books like Halfling units and ogre conscripts.
Anyways, I don't think you could go wrong choosing an Empire. Right now the empire seems to works decent with the current Army book although there's always room for a new book/models/items. Good Luck!