I thought Pen and S-La KW were about the same service-wise. Rooms were nicer at the Pen and not so noisy. We enjoyed the included afternoon tea/dinner snacks at the S-LA but they weren't as impressive as e.g. the ones at the Horizon Club in KL. I would go back to both hotels but have always preferred the (old) Four Seasons or the Sukhothai to either of them.
We did NYE at the Pen a few years ago and it's a nice spot to see the fireworks on the river.