IMO I think the important question is: what style of combat do you prefer? If you like guns and artillery, FotS. If you prefer swords and bows, RotS. The two eras play very differently and if you're not into the style of warfare the campaign loses a good part of its attraction. It's not as enjoyable when you need to play battles you find boring, or skip them all with auto-calc.

If you have no preference, RotS is cheaper by a good margin and should keep you busy until FotS hits the summer sales.