The Centurion and his horse questions.

[D B Campbell]

... I'm not sure how referring to an equestrian amongst the infantry could possibly mean a regular Centurio, which is what is being discussed, right?

I think you misunderstood the Statius quote. He's talking about a centurion and calls him "a horseman sent amongst infantrymen". The problem is that legionary centurions were virtually equestrians (some were already; others would be promoted on achieving the rank of primipilaris). So Statius may simply mean "an equestrian sent amongst infantrymen".

... riding is the questionable position- marching is the default. I'd say the cons are that none of the pros so far is particularly compelling.

I'd rather say that neither position has provided the killer argument. Many centurions came from the ranks of the equites singulares, so these ones were already accomplished horsemen. (Maybe that's why Julius Rufus was selling his horse -- maybe he'd just been promoted from the guard! did he already have a horse, or did he have to buy one?