My fiance and I discussed that before we filed for K1. We asked friends. Then/before, K1 is faster. That's why we are filing for K1. But then, K3 visas, especially if processed in Vermont is picking up with K1 visas now. The timelines for both K1 and K3 are almost the same. According to USCIS/NVC, both take 9-12 months.
But then it's a case to case basis. We filed for K1 in Dec '07. I had my interview last July 1, now waiting for my visa.
I hope you'll get the answers you are looking for. May you be guided accordingly. Either way, goodluck! May God guide you also.