Personally, I would go with the Myka. There's not much quality difference in the Suntour fork on the Myka vs. the RST fork on the Cannondale, but you seemed to like the fact that the Suntour could be adjusted to be 100mm. Also, with the disc brakes on the Diamondback...on low end disc brakes, there's usually no benefit in getting them over a set of V brakes. Go back and ride the Cannondale and the Myka again, maybe going over some curbs (up and down), and see if you can't get a bead on whether one feels better than the other because the rest of the components on the F9 and the Myka are almost identical. Good luck. 
