Amsteel is a product name that Samson uses to describe their lines made of Dyneema. Dyneema is actually very similar to Spectra and both are UHMWPE (ultra high molecular weight polyethylene). Dyneema is a product from a Dutch firm whereas Spectra is made by Honeywell. I’m not sure what you mean by stretch factor, but Spectra actually has a lower elongation than Dyneema. In practical terms we’re talking about single digit percentages that no FRC team will every notice - especially over a short length like a lift or a climber.
Each manufacturer tends to use a slightly different technique, they may weave the ropes differently, or use different coatings in their process depending on the application, or add an anti-chafe cover, or make the line ‘hollow’ so that it is easier to splice.
On an FRC robot, Spectra and Dyneema will be nearly indistinguishable from each other. Either will be significantly stronger than 550 cord for a given diameter. And now I’m done nerding out about that topic.