What can I expect for the conditions? How does broken top compare to South Sister in terms of level of difficulty for late Winter / early spring?

Expect a lot of snow for both and limited access. Southy is just a slog, not much difficulty. Broken Top has a class 4/low5 summit pinnacle so it's definitely more difficult especially if there's much ice/snow near the top. March is a good time to hit up one of the couloirs on Broken Top, when it's cold, like 9 o'clock or 11 o'clock. You can find both on summitpost. 9 is less steep and somewhat short. A good introduction to couliors.

Also Broken Top has some good backcountry skiing on it if your into that