Is the Trek a steel frame? If so, I'd go with the Trek. Yes, I am biased towards steel in general. In this case, I'm especially biased because of the greater twisting and other forces a tandem puts on a frame. Steel is much more tolerant of such things. Some metalurgist will now probably tell me I ma full of s***, and Lord knows there are tons of aluminum tandems out there, but assuming that the bikes are fairly close in overall quality, I'll take steel over aluminum every time, especially on a long bike.