craigm, I researched both Serotta and Spectrum and ultimately chose the Tom Kellogg-designed Spectrum after extended discussions with both firms. I had extensive experience with steel and love it as a frame material. The reason I opted for Spectrum was because the purchasing power of Merlin (whose custom shop does the build) is absolutely enormous. They can obtain materials that rivals the aerospace industry. Tom Kellogg specified some monster dimensions and this very unique frameset complete with Reynolds UL fork has been an absolute dream.
Serotta seems to have wandered astray of their roots and didn't seem to adhere to the customization and overall sales process quite the same as the two gentlemen (Jeff Duser) at Spectrum Cycles.
Both companies are based in the eastern United States and you could board a flight to visit with them both over the course of a few days. They both manufacture OUTSTANDING bicycles. My feeling is that Spectrum literally "goes to eleven" while Serotta is still wonderful even at a "ten" on the scale.