Jet driver here …
Contrails form, or don’t form, based on local conditions. While the altitudes at which they may form vary from the high 20s to the high 40s (thousands of feet), at any gven location they tend to form in a fairly narrow altitude band, 3-4,000 feet deep being typical.
Within the contrail-forming altitude band all jet aircraft are going to form them about the same, and all will be equally long lasting. The stealth aircraft contrail prevention system doesn’t seem to work and apparently they do just what we used to do, which is …
In the fighter business, the contrail altitude band was part of the daily weather briefing. You’d pay careful attention not to get into that altitude for fear of being visible to the enemy for a hundred miles. And since you never trusted the weather guys, you looked over your shoulder regularly, not just for enemy aircraft, but also for your, or your wingman’s contrail.
On-board radar and AWACS and data links are nice and all, but nothing beats seeing the other guy as the moving end of a bright white 20+mile long streamer while he’s still 80+ miles away and I’m just a grey dot far smaller than his eye can resolve. But I’m a heavily armed grey dot closing at 1200 mph with murder on my mind. He’s got about 3-1/2 minutes left to live …
As to the OPs question about different altitudes and some aircraft conning and others not …
Jets in cruise now operate at 1,000 foot altitude intervals over the US. If the contrail altitude range over your head happens to be 34,000-37,000 today, there would be a lot of traffic at 38,000 and above, or at 33,000 and below that would not be conning.
From the ground, 34,000 feet is only 3% farther up than 33,000 feet. Further, if the jets are 45 degrees above the horizon as you’re looking at them, the distance to them is roughly 9 miles total. Can you reliably identify the difference between something 9 miles away and 9-1/4 miles away?
And most often your look angle is even shallower than 45 degrees, making the airplanes even farther away and the 1000’ altitude difference an even smaller percentage of that distance.
Finally, now that almost all airliners are the same shape, you can’t even rely on apparent size to tell you how far away they are. You might be looking at a 737 20 miles away or a 777 42 miles away. They’d be the same shape and same apparent size.