Hi wellswang!
Congrats on getting a c3100 and welcome to OESF - I am also a new c3100 owner who is wondering what ROM to go with. Capn_Fish did a good job of summing up those roms but I'd like to add a couple of points.
I've been experimenting with pdaXii13 on the c3100 and its not quite usable in its current form. If you use the standard c3000 kernel you can't make permanent changes to files on your Microdrive or install any new apps as there seems to be a problem with the mounting of the MD right now (or something) and so you will lose all your MD saved/altered files after a reboot- and trust me you WILL need to reboot at some point Alternatively you can install pdaXii13 with the c3100 kernel from beta 3 and then you can save to MD and have it survive a reboot but this introduces a whole new set of bugs which I listed in my 'pdaXii13 on c3100 Install instructions and bugs' thread.
Saying all this, Meanie also owns a c3100 and has managed to quickly fix 100's of bugs in pdaX on c3000 recently and so I would hold on a little longer as I think pdaXii13 will be the best c3100 OS pretty soon. Maybe the next (final?) release will solve these problems encountered by c3100/3200 owners.
I've not tried r121yet as I'm all set on getting pdaXii13 running because r121, like OZ, is based upon Linux 2.6. 2.6 will give you a faster, more responsive machine overall but sadly 2.6 currently lacks c3x00 bvdd-alike video acceleration, which is essential if you're like me and want to use your Z for watching the odd video or playing emulators. r121, as far as I'm aware, can't be run off the Microdrive yet which would make it even faster.
Hope that helps!