Hey,
Slalom boards tend to be more technical to ride, You won't find it as easy to ride as the X-cite(awesome, still love mine). If your going to run with an 8.5 keep the 120, I run my 8.5 Helium on mine and it's a treat in light winds....It's a good flat water blaster with a 7.2.
You might want to start looking for your next board, you will get better quick and when your out in high wind you'll need that smaller board, at your weight you'll be looking at something around 70-80 ltrs. If you can blast around on a 120, you would be able to ride a sub 100 no problems, It's just when the wind drops off, or you stuff your tack or gybe that you'll have problems...but It'll force you to get better.
If coin is not a problem keep the 120 for another season as your light wind board, but get your improver for next season, If your just looking at flat water or bump and jump why not try a freestyle wave, When things get hairy these boards really perform. I've just picked up a JP 112FSW Pro and it's awesome, It's much smaller in volume then it states and it planes super early, I can actually get it going in less wind and with a smaller sail then the 120...but it's harder to keep on the plane in marginal conditions.
After next season your newly acquired 100-110 would become our light wind board, you'll sell your 120(good resale on the X-cite) and buy a 70-80 ltr for days when it's nuking. Then the following year it'll be a sub 70 ltr and you'll be popping loops everywhere, Then you'll have the perfect three board combination.
There's nothing wrong with slalom boards, but I just find the FSW boards much more versatile, If your going to be a speed guru or race maybe slalom will be right for you....But if it's just freeride go the FSW, confused yet.
Have fun...
Jase.