I can DEFINITELY answer that question for you. Get the 150.
We have pretty much the same boat (24SSL) with a 115. If it's just my wife and I, and our two kids (13 and 9) on the boat, the 115 is PLENTY for tubing. With a full tank, the four of us in the boat can hit 33 MPH, and 30 mph with the kids on the tube (which is scary fast for them).
BUT, if we have one more family with us, and especially two other families, the 115 is NOT enough. Tubing sucks big. Kids have no fun. With a full boat, full throttle on the 115 gets you 20 mph tops after a couple minutes of accelerating in a straight line, and turning slows the boat quickly to the low teens. It slows so much in a turn, the front of the tube nearly submerges, and its very hard to swing them out of the wake.
If I could do it over again, I would get the 150. At the time we bought it (last fall), the Verado with digital throttle and shift was the only Merc 150 option, and it was a LOT more than $2700. With hydraulic steering, it was in the $5-6k range last fall. When we test drove the boat with the 115 and hit 35 MPH, we thought the 115 was plenty. We thought wrong.
Then Mercury's new 150 comes along in Feb (Doh!) and it is MUCH cheaper than the Verado and If I could upgrade to the new 150 from my used (~30 hours) 115 for $4000, I would do that in a heartbeat. Spinzone is looking at the same upgrade and is seeing $5000 for the upgrade. So, $2700 is a no brainer in my mind.
One other thing, and it may be too late or not interest you, but we compared the 22SSL with the 24SSL and the 2 feet of difference is pretty major. You go from 11 to 13 passengers (if I remember correctly) and with almost everyone we know having 2 kids (so 4-person families) we could only take one family with us with the 22SSL. With the 24SSL and its 13 passenger limit, two families is no problem. That was key for us. So, just mentioning it in case you haven't considered that.
On fuel economy, we burn about 2.5 gallons per hour with just the 4 of us, and that's with lots of tubing (say 75% of the time tubing) and rarely using full throttle. With a full boat of people, its full throttle tubing and the burn rate is about 5.5 gallons per hour (and there's not enough HP to boot).
Get the 150! Or you'll be posting stories just like mine.