I will choose the SL3. I has bigger panels and also a 10" woofers. It is a great sounding speaker. Let your ears decide. Both models have over 15 years to be into the marketplace.
Clarity's bass/panel integration is probably better than the SL3's, but like Roberto said, the panels are bigger. Clarity would also be easier to drive than the SL3. How big is your room and what amp will you be using?
Current Clarity owner, Clarity can sound very good in smaller room, but it's very placement sensitive and small sweet spot. However, if your room can accommodate SL3 and seller already replace the panel, go with SL3 for sure.