For connecting two micro which one is better, UART or RS232?
UART
Cheaper, since you don't need any extra hardware.

EIA-232
(the name RS-232 is obsolete.) Will give you better noise-immunity, because it uses higher voltages and has lower impedance.

EIA-422
has an even better noise-immunity, thanks to the balanced signal, but unless your environment is extremely noisy (think high power electric machines, like several kW) you probably won't need it. You would need differential transceivers, and twisted wire cable.