whats the best key for mining?
and whats the best armor and how much it costs?
Gold key.
you can sell me your plat keys for cheap later on. 
All are worth buying except the platinum ones
Bronze is the best overall but it takes a long time to make time into platinum ones
I usually buy 100 silver keys then once I reach platinum, I just sell them.
Easy bytes 
What I do is buy bronze keys (atleast 2) use them, get 1 silver key, and then you get 200-300 bytes profit.
If you just wanna boost your byte and not really wanna collect the gems then Bronze Keys are a good choice. Its also more direct since the only thing you gotta sell is the silver key even without selling the gems you did get (no wandering around finding mgem buyer)