Have a read at the Forth Jubilee cruise thread - especially the bit when we motored through the Kincardine bridge into a monster wind vs strong current lump.
The group I was with had an SR4 , myself at 5m and a 6& a bit m rib. As the river widened as we motored downstream of the bridge the wavelength & shape changed. Betwween the three of us, one was always slapping the next wave (hard), one riding peak to peak like a magic carpet and the other "going up & down the hills".
What I'm saying is that there will always be a sea state that makes your particular boat uncomfortable or hard work. Yes, you could argue a longer boat is more likely to straddle bigger waves, but it's a big complex equation with the real definition of "a difference" lying with you & your crew's perceptions.
That probably wasn't much help tfior your decision, but if all other things were equal as you are implying (you'll probably be carrying more weight as fuel than the weight difference) and it was still going to fit the place I wanted to park it, yeah, I'd go up 0.5m for the deck space......
