How is playing the alto sax in Eb different from playing the tenor in Bb?

ive been playing my alto saxophone for like 4 years now and my high school band teacher just told me that if i wanted to move up in band to 2nd band from 3rd band that i would need to learn a tenor or bari but i dont want to learn to play a bari becuz i like tenor so i decided to try that but just as a question to any one who knos is that people say that its hard to switch to tenor becuz its in Bb and i dont understand the difference.is the s diffrent or somthing. so basicly im just wanting to kno the diffrence between playing a Eb instrument and a Bb instrument.