IMHO, the 5903 is much better balanced than the 5906. The all steel 5906 is overkill for the 9mm cartridge. The alloy frame on the 5903 makes the pistol lighter and, for me, nicely balanced. I was lucky enough to have both in very, very nice condition. After handling and shooting both, I sold the 5906 and kept the 5903.
Good luck.