I had ARC 2 so was in no rush to move over to Sonarworks initially, and spend more money on duplicate software.
After having tried both, Sonarworks is a no-brainer.
First, for speakers, doing the profiling is SO much better. It sends out a pulse to locate the microphone instead of relying on you placing the microphone in a specified position. Right there, Sonarworks is worth the outlay, IMHO. You are able to get a better profile, faster, and easier.
Also, the resultant profile sounds better. My ARC 2 corrections always sounded a little dull, and I always had to bump the levels up 4 - 6 dBs to get the same apparent listening levels. Sonarwaorks sounded good and balanced out of the gate.
As for the headphones, I have the KRK 8400's which are very nice cans for the price. With Sonarworks they are a total bargain. Since I live in an apartment in the middle of San Francisco (dense housing - surrounded by neighbors) and am a nightowl, being able to reliably mix on my KRK's is very convenient.
All in all, I am very happy with my purchase and would definitely recommend Sonarworks over ARC 2.