Hello
I'm thinking about wireless connection between 2 arduinos
Can you tell me which wireless communication module is better to use?
I need about 300m link.
I was thinking about transparent uart modules (like APC220 from ebay)
or maybe zigbee will do it better?
I need about 300m link
That is a long way. better sort out the transmitters and receivers before you worry about protocol.
Is this indoors or outdoors, mobile or fixed?
outdoors in line of sight 
i dont need hig baud rates, I need it to be easy to use and reliable
The XBee pro modules can handle that distance. They are easy to use, too. Not cheap, though, and fiddly to get configured, unless you get the right ones. (Hint: Get the Series 1 versions, if you just need two Arduinos to talk to each other).
mmm...
How about apc220 from ebay? Did anyone use them?