So I need to add a volume pedal to my rig, and have been looking at the ernie ball one as they seem to be pretty popular. My guitar has active EMG's, so I would need the 25k as opposed to 250k model, right? Or is that only for preamp stuff?

Also, I've heard that the string in the ernie ball volume pedals breaks pretty frequently and is difficult to repair. Any credence to this? And is there a better pedal I should be looking at?

You can get either. I have EMGs and a passive vol pedal, and the difference is negligible. Just make sure you get a decent one, or you'll notice a considerable signal drop. I looked at the Ernie Ball JR volume pedals and the 25k paled in comparison to the 250k non JR version. Overall, just try them all out, and don't skimp too much or you'll regret it a little.
I'd go Ernie Ball, personally, but try them all out b/c some seem to be budget gear. The better ones are larger (hence the JR models being cheaper) they also make amp blending expression pedals and wahs, so don't get those confused. Pay attention to the potentiometer ratings, they are what will determine the fluidity of the volume swell/sweep. TRY THEM ALL OUT, you'll get what you want and won't regret it.

Outside of EB, I've seen Dunlop makes a volume pedal, but I've never used nor seen it in person so I can't really comment, but I would imagine its decent, based on their other stuff I've used.