A lot of people play both. I've played tenor for 22 years, and alto for 18 years. Because they are in different keys, the music is not interchangable between the 2. Many tenor players also play soprano instead of alto because they can use the same music. (same key, Bb) I play mostly by ear. In fact, I haven't read music much in over 12 years now since I last played with an orchestra. The different keys really don't bother me too much. It's easy enough to transpose 5 half-steps. The alto and tenor play very differently though. The tenor emboucher is much more forgiving than the alto.