I play bassoon duing concert season, and trombone during marching season and parades. Currently, I'm played the school's old Yahmaha and it works fine. I played the Bach Stradivarius for a week and I liked it. A friend let me try his Blessing and it had a sweet sound.

Not sure if this'll help, but I realized that I can produce a better tone on the upper range (due to my tight embrouchure on the bassoon) than the lower notes. That and I can produce a louder sound when needed easily.

So I was just wondering what kind of horn you guys think I should get?

Well I'll just throw it out there that when I went to big bore horn my low range got HUGE compared to before. My mid range filled out nicely and my upper register never changed. Big bore is not for everybody so try a few, but keep bore size in the front of your mind.

.547" all the way...

Keep in mind this is NOT a full time bone player. I do all my Navy marching jobs on a large horn, as do many of my coworkers. But we play ALL THE TIME!. A .547 horn is too much for many high school players (though they'll never admit it out of pride).
My recommmendation for a marching horn would be a student yamaha (YSL-354). They play almost as well many upper grade horns but can take a beating on the field.

Ok, thanks Steve. I'll look that up.

And TromboneMN, um... I think I might have to agree with Steve on the bore part. But thanks for the advice.

I play a YSL 354 and i love it

if you have trouble with low notes get a bass trombone. I play the saxaphone also and playing bass helpes me with low notes on regular

I have to respectfully disagree with you on that point. If someone isn't ready for a large bore tenor, they sure as heck aren't ready for a bass.
Problems in the low range can be an embouchure issue or an air issue, or a combination of both. It could be other things (poor slide placement, posture, etc) You have no way of knowing without seeing or hearing her play. So to recommend switching to bass at this point is honestly a bit careless and misinformed. Sorry.