For voice, they are about even. For data, the S3 tends to pull a stronger signal. My wife has the S3, and she is constantly on 4g while at home, while my phone would either stay on 3g, move slower than molasses, or not move at all while connected. The LTE chip is literally my only real complaint about this phone. However, Sense runs better than TouchWiz, in my opinion.
The choice to stay or leave is up to you, but don't make data the reason for you to leave, unless you must.
Phone: HTC EVO 4G LTE
Sense 5, S-Off
Tablet: ASUS Nexus 7.2
Rooted, Custom Rom & Kernel