Which is better for tempo runs and races?

The Magic Speed 5. It is lighter, has a snappier FF LEAP toe-off and a more aggressive rocker, which suits hard efforts. The Speed 3 works for tempo runs but feels less quick.

Which has a roomier fit?

The Magic Speed 3. Owners of the Speed 5 report a narrower fit with a tighter arch and toe box, so wide-footed runners should size up or try the older model.

Is the Speed 5 worth stepping up to from the Speed 3?

Yes if you want less weight and better traction. The gains are real but incremental, and the fit changed, so try before you commit.

The Speed 5 should hold up better on the outsole thanks to ASICSGRIP rubber. Foam life is similar on both, and plated shoes are best kept for faster days.

Do both have a carbon plate?

Yes. Both use a full-length carbon plate in a neutral, 7mm-drop package, so the ride character is similar.

Which is better for wide feet?

The Speed 3. The Speed 5 runs narrow for many owners, and neither comes in a wide width.

ASICS Magic Speed 5 vs ASICS Magic Speed 3

Buy the Magic Speed 5 if you want the newer, lighter plated trainer with a snappier toe-off and better traction. The Speed 3 is the older generation, still a 7mm-drop carbon-plated neutral shoe with a similar stack, but it is heavier and its fit and foam are a step behind. Reviewers of the Speed 5 split on fit and perceived build quality, so try it on if you have wide feet or a high arch.

Choose the Magic Speed 5 if you want the lightest, most responsive version with a full-length carbon plate and grippy ASICSGRIP outsole.
Choose the Magic Speed 3 if you already know the older fit works for you and want the proven previous-generation plated trainer.

Prices

ASICS Magic Speed 5: $128.00 at Pace Athletic (checked today)
ASICS Magic Speed 3: $129.99 at Amazon (checked today)
Difference: the ASICS Magic Speed 5 is $1.99 cheaper than the ASICS Magic Speed 3 today