Depends how much surfing compared to flat water u would like to do. I'd say the 10'6 Naish if u want to do a bit of both.

Just wait for DJ to finish work and he'll post some more advice on here

The Starboard 11'2 has nice glide and sweet wave riding abilities all wrapped into one. Its 30" wide but doesnt have the chunky rails like the Naish. The 11'2 has a more surfboard outline, with thin rails and a square tail.
Mandurah doesnt really get that much swell during summer so i reckon the 11'2 would be fun to cruise the reefs and pick off anything from ankle high waves to head high bombies.
Halls head would be a fun place in winter with the rolling waves breaking down the point and off the sand bar.
I can think of many places for SUP surfing around mandurah that are usually not surfed. The options are endless.
Then of coarse there is the estuary.
I will probably come down your way in a few weeks. You can try my 11'2... You will love it

Starboard 11'2 is very adaptable. Good fun surfing, longboard style and plenty of float for flat water. I havn't tried JL's. Naish 11'6 felt unstable when I tried it but that was only my second attempt at SUP.

I've got a 2nd hand 11'6" Kalama which I could sell for $800 for the board if you are keen? Drop an email.