I had Pfizer, but got boosted with Moderna because that was all they had. Otherwise, there was like a two-week wait to get an appointment.
Arm was a bit sore for a day or two, and felt a bit "off" for a day. Upset stomach mostly, sort of like you might get if you were starting to come down with the flu. But less of a reaction than when I had my second Pfizer shot when I had low-grade fever and chills at night.