Hi Joao!
An interesting decision! I have both, and have had decent results from both. It depends more what you are looking for. I'd say the Agat is more fun, it is an unusual and quirky camera. The Chaika is more conventional and probably ought to get better results, but a lot comes down to luck with these old FSU cameras!
Here are two shots, the statue is with the Agat, the mural is with a Chaika II.
Have fun!
Michael.
http://www.ussrphoto.com/UserContent2/27122022_36217733742_0f45017cdb_k.jpg
http://www.ussrphoto.com/UserContent2/27122022_36217733742_d079f9af06_o.jpg