From my experience, fiberglass sousaphones aren't anywhere near as bad as their reputation says they are. About 15 of them mounted on Ohio University players came right in to WVU's stadium a couple years ago and plastered our 24 brass sousas all over the place. Our director was livid, as he should have been. We played like crap. Judging from other instances, fiberglass sousas do not hold up to overplaying AT ALL. That's when they sound horrible. So it's all in how you play them. If someone doesn't sound good on a fiberglass, they're not gonna sound much better on a brass sousa.
Fiberglass horns seem to hold up to the "rigors" (read: abuse) of marching band a bit better (no dings). However it would seem that the same behaviors that destroy brass horns eventually will catch up to a horn made of anything else. Equipment needs to be taken care of.
Another thing I've noticed about sousas and convertibles is about the bore. The bore on sousas is a bit smaller in most cases, not bigger. Also, and more importantly, the bore is much less conical than convertibles. That makes a big difference in sound and power. Convertibles get a much rounder and fuller sound than sousas, which have more projection and "color" to the sound. Another consideration for sousas is their visual contribution. They're physically bigger, so their contribution will be bigger too.
I love both instruments. A good convertible or contra bugle sound is like butter to my ears, and I've also never had any marching experience as fun as the "rock" section of Country Roads in WVU's pregame. One should consider a band's style when deciding what type of horns to get. Shoulder horns simply wouldn't work at WVU. The size of the sousa's bell contributes too much to the show, and I think with all the trumpets and trombones in the group the bass line needs to cut through to match all that brilliance in the sound. With a band that emulates more of a corps style sound, shoulders or convertibles are the only way to go.
I love topics like this. Makes for interesting conversations. For another interesting topic or two, consider the marching band where I serve as brass tech. We don't use trombones. As a trombone player, I kinda dislike the move and also kinda do. The director just doesn't like the sound, prefers the darker sound of the baritone. We also use flugelhorns, requiring some range accomodations, in place of mellophones. I like the way the mellophone fills its role, but I don't really like the sound or how difficult they are to tune. I'd like to check out alto horns and see how they work but they aren't anywhere near as popular as mellos.