Avoidance vs. Riding

Addama

Which gives more damage mitigation: Trans Avoidance, or Trans Riding with a warhorse?

Sena

Avoidance gives infinitely more, since riding gives no damage mitigation (it's only a dodging bonus).

Blujixapug

Avoidance gives IIRC 12.5-13% cutting/blunt resistance at Trans. The benefit is gained based on total lessons learned towards trans, so investing 870 lessons will get you 6.25-6.5% cut/blunt resistance, while learning from Inept to Novice (7 lessons) will get you 0.05-0.052% cut/blunt resistance.

I am not sure exactly what the Riding Defence ability actually does. I've never tested it and I'm reluctant to assume it gives actual damage resistance.

For calculating your chance to dodge:

Being mounted gives the equivalent of +1-4 dex based on skillrank, and +0-10 dex based on your mount's level (apparently very rarely above +4). I am unsure if this is tied to the Riding Defence ability.

Avoidance gives the equivalent of +1 dex/skillrank, so +12 at trans.

(Speed elixir gives +4, drunkensailor gives +4, evadeblock gives +3, acrobatics gives +3 mounted/+6 unmounted, and weaving gives +3 mounted/+6 unmounted.)

Those dex figures are from like a decade ago, so I'm not sure whether they are still accurate or may have been modified by any changes. Dodging is such a weird, unintuitive thing that I wouldn't be surprised if it was phased out.

Sena

The dodging bonus from riding does come from the Defence ability, before you learn that you get no dodging bonus.

The rest of the information should still be accurate, except that the Limbcrusher trait reduces the dodging bonus from riding (including both the riding-based part and the mount-based part) by around 30%.