If you're going to be doing a lot of two-up riding, I'd suggest going with the 300cc engine, for more power.
You being only 5'7" doesn't really tell us much except that you'll fit quite comfortably on the Yager. If you're not overweight, I'm making a guess that you're about 150 pounds or so, and if that's accurate, the Yager's got plenty of power for that weight. I'm about 185, and with my backpack, I'm about 195 pounds.
The seating is pretty comfortable for two-up riding, as long as you're not doing a long ride. Even then, stopping and stretching every once in a while should set things right.
If you do a hard break-in of the engine, you'll get better ring seating, and thus less blowby, and thus a bit more power. My Yager, when it was still tight, would hit 79 MPH (although it was way over the red-line). Now, after it's started loosening up, it'll still do 72 MPH, and I expect that'll drop to about 70 MPH as the engine finally gets loosened up completely.
When the engine needs a rebuild, I'm going to be using gapless rings, which should give very little blowby, the oil will stay cleaner, and the engine will have a bit more power and efficiency. But that's waaaaayyy out in the future, as I've still got less than 3000 miles on it.