BB,
Moved your question so you'll get more feedback. My $.02: I see you started with the Streetbob and picked up the SERG in April. If you don't ride the bob you'll never ride the K-9. I rarely ride my Titan now that I have the SERG even though I love it when I do. I'm 5'11" and whiled the SERG feels roomy, the Ultra feels cramped (to me).
The question is, "What kind of riding do you do?" I like the SERG for it's versatility. I can tour with the Ultra and whip is around corners when I want. Having said that, no bike is too much for you. Woman ride Ultras with technique, not strength. (Talk to Vgirl). If a touring bike starts to go down, even in a parking lot, few of us have the strength to hold it up. An advanced class is always great to give you more confidence on your bike (and teach you limits).