Brake selection sounds about right to me for how you describe yourselves and your riding experiences.
As compared to the Formula, the Avid should perform just as well and you should have no trouble servicing it yourself.. adjustments, pad replacement, cable replacement.
Tandems made from different materials are no different than single road bikes make from different material: the handling varies more due to things like the frame design, tubing selection, construction, steering geometry and wheel / tire selection than simply the frame material(s). So, if the baseline is your Santana, it's hard to know without knowing who's carbon frame you're buying (Calfee, Santana's, AriZona's, or ???) and how you may have spec'd the frame, extra light, extra stiff, etc... I'd also have to know more about your Santana such the year it was made, which fork and wheels you're using, etc. before offering an opinion.