Depends where you ride, if you ride mostly flat water you can get away with a flatter board. Also if you ride in lighter winds then a board with loads of rocker will kill your power and mean you never get to ride enough, so again in that situation a flatter board might be more for you. If however you ride in any amount of chop, or the wind is strong where you are, get a board with some decent rocker (at least 4cm) cause it just makes life in boots that much better
How much do you weigh? I'm 70kg and ride a 137 with boots all the time. If you're 80kg+ then look at 140cm+ board
What board are you riding now?? I just chucked boots on my 131 thorn when I first tried them, loved them so I bought a dedicated boots board, but if I didn't like it I woulda just sold the boots. You could do the same, although you do run the risk of snapping a strap board if you boot up...
pros: more powered wakestyle riding, super solid landings, feet won't come out as you throw the trick, much more satisfaction landing a new trick than in straps cause you have to work harder for it
cons: spend 30 seconds extra getting your boots on and off at the end of a session