Ubuntu is more stable.because they give more emphasis on Ubuntu Project and GNOME(I like it

)
regarding black screen,ur display driver may not be installed correct(for eg:nvidia).
you can reset ur passwd by pressing "e" in grub ubuntu entry-again press "e" on the kernel line.now remove the "ro" to "rw init=/bin/bash" and press enter.
Code:
kernel /vmlinuz-2.6.20-16-386 root=UUID=whatever rw init=/bin/bash
now press "b" to boot.when it enter's the root prompt "#",u shud enter as below:
^enter ur new passwd and exit reboot(write down ur new passwd if u forgets easily or enable autologin in gdm)