Really, this is an apples vs. oranges comparison. As mentioned above, the CJ and ARC sound is about as different as you can get, and both are pretty much the models for their style. I suggest that you try and audition the ARC with your speakers and see which of the two amps speaks to you. I'm certain that one will very quickly stand out as a preference.
I upgraded my system last year and spent quite a bit of time auditioning both of these amps. In my case, it didn't take very long for CJ to stand out as a clear winner. Between the two brands, I found the CJ equipment to be much more emotionally involving, as well as being easier to listen to for long periods. To me, the ARC systems felt somewhat cold and lifeless. However, in every system I listened to, the CJ amps really seemed to be at their best when paired with other CJ electronics, while the VT-100 sounded good with a variety of preamps and sources. Just something you might want to consider when evaluating the two amps for your system. Anyway, it boils down to a personal preference, but it would certainly be beneficial for you to hear the ARC before you decide if swapping out the CJ constitutes an upgrade.