5'8" or 5'10" Cab Subwoofer

Rider: Weight 82kg, Level intermediate
Style: Freeriding, Surf (not very often in the surf)
Disclosure: regular punter who has to buy his own gear AND hide it from the missus

I'm wondering whether to get the 5'8" or 5'10" Cab subwoofer. I'm leaning towards the 5'10" as I'm 82kg and usually riding with a 12m in flattish water (freeride) with occasional windchop generated swell in Norway, if I'm lucky the 9m will come out a few days of the year. I usually head home to Gero for a month or so of the year so I'll use it in the waves a bit there, but i predominantly free ride on pretty flat water with a little chop.

I'm mainly looking for a board to get out when the TT doesn't cut it so I'm thinking the more volume the better. If so I'll get the 2014 which offers the 5'10" otherwise i'll save some loot and get the 2013 5'8". I'm sticking with Cabrinha as there are bugger all local shops that sell kite surfboards here in Oslo and my local will let me demo a board.

Anyone have any insight for me?