Damn that's a deal.

If funds are tight, BDop's DIY wheel kit is also a great deal:

DIY Alloy Wheel Kit 20/24

Not quite as light, but the BDop kit uses brass nipples, whereas the BHS one uses alloy nipples, even on the rear (were I to build that kit, I'd swap in brass nipples on the rear, and maybe even on the front).

So yeah, I'd also recommend wheels. An extra set of wheels is a lot more likely to be useful than an extra crankset. Or you could sell your old wheels for a decent return - no one's going to pay much for a used FSA crankset...