Which has the longer deck?

The P1, at 47.2 inches against 43.3 inches — and it is also 0.8 inches wider at 17.3 inches.

No. It caps at 3.72 mph. If jogging matters, the Sperax is the only one of the two that reaches it.

Which is better for a small apartment?

The P1, because it folds in half. The Sperax keeps its full length even with the rail folded.

It folds down onto the deck, which is enough for storage against a wall, but the frame still sits higher than a bare pad.

Head-to-head

The P1 is the longer, wider deck: 47.2" x 17.3", folding in half, quiet, and speed-capped at 3.72 mph for walking only.

The Sperax trades four inches of deck length for a folding handrail, a 300 lb stated capacity and a top speed more than double the P1's.

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Manufacturer-stated figures. Confirm on the Amazon listing before you buy — brands revise these between production runs.

Longer deck for taller walkers and longer strides

The best pure walking deck of the two — long, wide and storable, if you never need to go faster than a brisk walk.

Riser-bar model that doubles as a light jogging treadmill

More versatile and more supportive, but bulkier and harder to slide under a desk.

Which has the longer deck?: The P1, at 47.2 inches against 43.3 inches — and it is also 0.8 inches wider at 17.3 inches.
Can I jog on the P1?: No. It caps at 3.72 mph. If jogging matters, the Sperax is the only one of the two that reaches it.
Which is better for a small apartment?: The P1, because it folds in half. The Sperax keeps its full length even with the rail folded.
Does the Sperax rail fold completely flat?: It folds down onto the deck, which is enough for storage against a wall, but the frame still sits higher than a bare pad.