Based on the machines I've seen at Westec and Haastec, I'd say that I don't think the chip flow is as good in the new ST series machines. The SL-20 just drops them down to the conveyor area. The ST has the tailstock rails running horizontally and there is a lot of horizontal area to accumulate chips.
With all of that said, the Y-axis options really seem to boost the usefulness of live tooling on the ST series. If a lot of your parts were having second ops done in the mill, I'd consider those two options (live tooling and Y-axis).
In fact, I'd consider the ST-30 with live tooling and Y-axis. It buys you 3.0" bar capacity and I'm intrigued by the possibility of moving small milled parts to the lathe. Imagine loading up a 5-foot bar and making a run of small milled parts without having to open the doors. Mill all the features, using the spindle as the C-axis and part them off at the end. This obviously won't work on all parts but, if you have control over the design of your parts, it opens new possibilities.