A reasonable starting point is to set the footpegs where you can "lock in" to the thighbraces by pointing your toes, but have some play when your legs are relaxed. Some people add foam to the area where their knee rests on the hull to adjust their leg position.
Cockpit outfitting is very individual -- it's very common for paddlers to customize their boats for a better fit.
The long demo is a great idea -- it's hard to get a good feel for a boat in just a couple of minutes. I suspect that the narrower beam of the 135 will be more comfortable for you than the 125.
Think of the cycling equivalents -- the 125 is more of a cruiser, while the 135 has slightly narrower tires, tighter geometry, etc. A bike like that might seem a bit wobbly at first but would offer more performance once you logged some miles with it.