Paceora The full picture
Apartment size determines the recommendation more than anything else. In a true studio, only the C2 makes spatial sense. In a one-bedroom with a dedicated desk area, the A1 Pro becomes viable because it doesn't need to move as often. The X21's upright storage is a creative solution but still takes floor space. The R1 Pro and X25 are generally too large for apartments unless the apartment is unusually spacious.
The Paceora angle
In a small apartment, convenience is everything. If the treadmill control adds friction (finding your phone, opening an app), the walking habit dies faster. Paceora keeps it to one click from the Mac.