I've got 303 tubulars, not firecrest. Had 404 clinchers, not fc, and just repalaced the 404's with new firecrest 404's.
I'd go with the 404's. Thy're more aero than the 303's and the weight difference is only 100 grams or so.
Only downside is that the 404's are a bit more subject to crosswinds. However, I haven't found it to be a concern in winds below 20-25mph.
The only reason I might favor the 303's is if they would be your only wheel, and you'd have to use them in high wind conditions, or you were buying them for a hill climb race.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.