I can't tell you about a married couple, but me and my dad use an older northface 3 man tent (It was very high end in the day, probably 6-7lbs). Fits us (200+ lbs 6'3"-6'4") and our stuff.
My two person (sierra designs) i use for solos wouldn't fit me and a small child, but then, i'm big.
I'd definitly go 3 person. I wouldn't spend double to loose a pound or two.
I'll tell you, i notice the difference between my two tents with a 2-3lb difference.