Played Tuba in the school brass band for around two years, first on the smaller Eb, graduating to a larger Bb. The valve fingerings are the same as the trumpet for a given note i.e. all open = C, first two = A etc.
It does take a little practice to develop the respiratory capacity to keep a Tuba going over a decent playing time. I still credit my good lung capacity to playing the Tuba for a good while in my earlier years.
The Euphonium is a more versatile instrument. Although it doesn't go as low on the register, it's a bit more flexible in use.