Thanks for all the responses! I suspect my problem is that I’m not used to switching and am taking longer than normal to adjust. Cathy mentioned cars with manual and automatic transmissions, and I used to switch between automatic and manual without thinking about it — well, there was that one time I went for the clutch on an automatic and started hitting the brake with my left foot ... fortunately I wasn’t going very fast (because in my mind I was shifting into second).
Patrice’s point about it being good for the brain makes sense to me — the more things we learn, the more easily we can learn. If I force myself to sew on a different machine, eventually I’ll get better at switching back and forth. It’s just going to be a little bit painful getting there, and I'll probably have to mess up a few small projects before I do.
I still have a long way to go in picking a second machine. I know I need to go in and test drive some and see how they feel. I might stick with a Husqvarna Viking, especially if I can use the same feet, since that would save some expense. I already checked the low-end ones and discovered that they use different bobbins — too bad!