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.
The reason that the dealer couldn't give you a hard and fast answer is that an angle that is heavenly for one paddler can be recipe for a sore back for someone else. I for example am fine with a pretty flat leg, I paddle with others whose musculature and joints are disposed to need a higher angle to be comfortable. The amount of stretch in your low back muscles as well as the flexibility right around your hip socket can all influence this.
To complicate it for you, if you are a bicyclist you probably have a little more flexibility thru your lower back and hips than most. So it's less likely that you'll get some of the red flags just from sitting in the boat that a stiffer person might hit.
There are arguments about what angle makes for the most efficient stroke too, but much of that get way more refined and scientific than you need care about for your purposes right now. A boat that is on the narrower side and that doesn't swallow you deals with that stuff OK.
There are two things that you can tell without a ton of time. The first is, when you bring your leg up into the braces (assuming the boat has 'em), are you hitting the braces with your knee or above the knee, with part of your thigh? You want it to be the latter, the more thigh the better. Pressing your kneecap into a brace for every turn or edge won't feel good.
The other is, do you have enough room to be able to drop your legs and completely relax them, just let them lie loose along the bottom of the boat? Over longer periods of paddling you'll need to do this to reduce the strain.