There are no feature differences between Rails Autoscale plans. The only difference is the number and type of dynos supported. See the Heroku Elements marketplace listing for current prices.
The "autoscale limit" for your plan will constrain the "autoscale range" you configure in your Rails Autoscale settings.
For the image above, you would need a Silver plan which supports an autoscale limit of up to 9 dynos. If you needed an autoscale range that extended to 10 dynos or more, you would need a Gold plan.
The Standard plans are for apps that only use Standard-1X and Standard-2X dynos. If your app uses Performance-M, Performance-L, Private, or Shield dynos, you'll need a Performance plan.
Heroku doesn't natively support trials for add-ons, so to take advantage of the free 7-day trial, you'll need to sign up for the Trial plan. The Trial plan supports an unlimited autoscale range and all dyno types.
After 7 days, you'll be prompted to upgrade to a paid plan to continue autoscaling. The add-on will remain installed indefinitely (with autoscaling disabled) until you choose to upgrade or uninstall. (An add-on provider cannot uninstall an add-on for you.)
You can also email a request for more time.
Starting in September 2020, each Rails Autoscale plan was split into a "standard dynos" plan and a "performance dynos" plan.
Existing customers are "grandfathered" permanently for their current plan. If you're using one of the old plans, you can stay with it forever. You'll only be prompted to choose a new plan if you decide to change plans.
New customers, or existing customers changing plans, can only choose from the new plans.