I also use sai and photoshop. I mostly use Sai because of the stabilizer gives me smoother lines and such. It is also must simpler than photoshop. However, there are a lot of people that I know that are using Manga Studio 5, but I found it a little awkward for me. The biggest problem I probably have with Sai is that it restricts my canvas size, limited layers (up to 200s), limited to your memory, and that it doesn't save transparent.
