Boeing Field has 3 FBO's to choose from, Signature, Modern Aviation (formerly Clay Lacy Aviation), and Kenmore, any of which can provide needed services. I personally prefer Kenmore, their fuel prices are lower, excellent service, but they are perhaps less "fancy" than the others. There is US Customs there if you need a port of entry, but I have no experience with that. The Customs ramp is between Signature and Modern, maybe closer so you only have to park once, I don't know. Whichever you choose, always a good idea to call ahead for details.
My only negative thought, December weather can be thick fog, but not often below ILS minimums, but, then, anywhere along the Pacific NW of US can be that way, on a bad day, an alternate may be miles away.
Cannot really think of any real cons against choosing KBFI.