The major difference I can see is body wood and the active preamp. So far, the vast majority of the reviews I have read on the Sire have been very positive. The Squiers have been knocking it out of the park for the last 3-5 years in quality control and value for the dollar, so the bottom line really comes down to whether you are a passive or a active pre-amp fan. I love the idea of an ash-bodied Jazz for the Sire price point, but I'm so much happier with the passive pickups of the Squier.
I have to admit that I'm intrigued by the Sire, and may yet pre-order one. Distribution is direct from the factory, and the first batch have all been sold out, due to great buzz and demand. You may end up having to wait a few months to get one.
You might want to consider pulling the trigger on the Squier now, and snagging a Sire somewhere down the road.