well, contra is a wonderful instrument, and sousaphone is a really bad instrument unless you are in new orleans (then it is the best instrument (besides bone))
well, in marching band i marched sousaphone last year (because the tuba players are cool and my section was being really lame (basically i was the only baritone that knew anything and they hated me for it so i made their lives easier and left))
sousaphones
1. are really bad sounding instruments it makes your tone sound a lot huskier than concert tuba or contra (which is concert tuba on your shoulder basically)
2. are confining, marching techniques are usually based on lifting the upper body off of the lower body and it helps ALOT to lift your head (solved with contra's 10 degrees up or 15 or 20 (which ever your corps does)) lifting the arms from resting on your sides to where the valves on the contra are.
3. destroy your traverse (slide) because you have no lift on your spine.
4. wobble because they are resting on your side (basically on your upper leg) and not on your upper body which is the part you are keeping smooth.
personally, i dislike the sousaphone unless it is jazz. The sousaphone should have never been let on the field in my oppinion.
so
when you get your contra, you will be like oh my god i hate sousaphone.
personally im sticking to baritone for corps, and quiting marching band. kill two birds with one stone