Interesting question here.
We have a bike (non-Peteton) and a treadmill. As I just hit 50, I have a hard time running consistently due to the pounding on my knees and hips (lifelong athlete with a number of old knee injuries). Let me throw a few observations/thoughts out here and I welcome others' feedback.
Pro-Treadmill: I "feel" like I can get a more well-rounded workout even by walking at a brisk pace (I try to go at 4.2 MPH). As I get older, I anticipate walking will better help avoid balance issues. I assume it will work more muscles -- back, arms, and shoulders -- than bike.
Pro-Bike: Insightful comment above at always being able to walk outside, so bike can be a good change of pace. Even at what I consider a moderate pace -- 20 MPH at 4 out of 10 tension -- my heart rate gets higher than walking at 4.2 mph (though I suppose inclining the treadmill will increase strain/effort, I might try that). But I just don't "feel" like I get as much of a workout on the bike.
Con-Bike: The repetitive motion of the bike is tougher on my knees than brisk walk.