On Yamaha, make sure you can play it in tune with others before you buy it. I've had people in my community band purchase Yamahas only to return them since they couldn't play in tune with the band.
They were able to play remotely in tune before hand <g>.
I recently purchased a Gameinhardt and I love it. It depends on your personal preference. My flute is a Jupiter and I wouldn't trade it for the world. It's a wonderful brand but Gameinhardts are "harder" to play accoring to some. Yamaha's are not my favorite but they are decent brands. Personally, I would go with Jupiter or Gameinhardt, my personal two favorite brands. =)
The general rule of thumb about student level models is that Gemeinhardts are slightly harder to play and have poorer tuning and scale, but you can get better sound out of it if you can overcome the gremlins. Yamaha makes their, well, everything, to be as easy to play the same as possible, but you do sacrifice a tiny bit of tone.
Between the two, though, most people get the Yamaha for a Pic because you're not playing jazz or ethnic or folk music on it. You want easy and repeatable results since you are the most noticeable thing in the entire band when things get out of tune(or just about - I'd rate the trumpets about the same).
Jupiter, I'm not that familiar with.
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Edit - I'd recommend a wood headjoint and a metal body if you can manage it - that seems to solve the tuning issues with the Yamaha.
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