I think it doesn't look nice to look the same as every body. To that regard, rarety is important but I think that's almost just about it.
Cosmetic is probably the most valued thing to my opinion. You always have things that are only there to be there (collector) in every game and that's super expensive all the time.
Usually rare things are very hard to get through RNG but that not the actual definition of the word "difficulty" you'll agree with that I think. I don't know many games where hard stuff to get aren't very few and the most valuable thing at the beginning to finally just be kind of casual.
About your question, I think I will always keep the cheapest and sell the other one. You mainly have 2 kind of people: - - those who get a little money and get back to poverty
-Those who always invest money and always stay posh
The second category don't spend money for nothing very often and wearing an expensive stuff who has a cheap equivalent is the beginning of loosing money. No matter how hard it was. Profitability's always first in multi-player gaming (which is quite sad).
The most important things you dismiss are 1) knowledge 2) lazyness.
You can always earn money in other's laziness or because you know something they don't. That usually how you get gold x10 on a game.
Doing hard stuff is not usually the best way to get richer. It's good. But not the most efficient at all. (unless you find a way to make it easier an quickly, which as I said is more about knowledge)