Don't know which one works best for your treadmill?
This might help:
Solid Step:
If your treadmill has straight handles and you would like something simple, yet effective to help you get into a walking practice.
Pros: Affordable, gets the job done.
Cons:
Only for straight handles, Does Not come with handlebar clamps,
3 themes to choose from.
Classic:
Lightweight, enjoy the convenience of a fast set up and removal
Great if your treadmill is smaller than 34”.
Classic Plus:
same as Classic; Plus, if you like more room and comfort while working.
Pros:
Can be used on straight or slanted handlebars (just specify it on the drop-down menu)
8 themes to choose from.
Cons:
Treadmill cannot be wider than 34.5”.
Pro-XT Plus:
If your treadmill is wider than 35”.
Pros:
Adjustable to your treadmill’s length (up to 40”)
8 themes to choose from.
Cons:
about 2 lbs heavier than Classic or Classic Plus.