It could be different to the point of changing the outcome of the next battle, for example: if he needs half his CS to fight, and the other half to collect resources, and they are all of the same rank, but have different experience, then choosing the half with more experience to fight would be the better option, expecting a higher proportion to rank up during the next battle. It's a detail, but in theory having sliders to filter those with HP, experience, etc, higher or lower than the chosen value from a selection could be a thing (I know there's the "injured" selection, but this could replace that, being clearer and more flexibly customisable).

Just experience doesn't change anything, it's only used for promotions. You can double click to get all units of the same type and if you triple click you can limit the selection to the same rank. Generally micro isn't of much relevance until you become really really good at the game, so I'd suggest to focus on macro instead.

When I play, I often (after a battle) try to select all of the soldiers with high experience. It always takes a big amount of time which often also results in me losing. So do we have a hotkey for this or should I simply just not try it because it takes so much time?

It's definitely a cross between Roman clothing and Parthian. Valerian and Shapur I look quite similar.

Banquet reliefs at Palmyra often depict young men in attendance dressed in almost identical outfits to their master. Their costume consists of a belted roundnecked, long-sleeved tunic with a decorative band on the neckline, cuffs and hemline, a cloak, tight trousers.