are includes and macros two different possibilites or just the same looked from two sites?
That's two different things, two different possibilities. And you can mix them together

it isn't also possible to programm a 12bit or 14bit pic from the mplab ide, like the 18bit family shared the code in several files
??? not sure of the question sorry

is free codespace and the circumstance that each call from the include is copied in the hexfile separately a disadvantage?
Once you compile the code with 1 or more Include... it's nothing else than a single big file... Only 1 HEX file is generated. No problem with it.

... i'm not sure which way i should go for further projects ;-(
There's no better way. Use the method you're confortable with. Try to always use the same method, same coding style and blah blah. That help when you return to a program done 1 month/year/decade/century ago.