I'm no flute expert, but we bought a more advanced flute for my wife some years ago and I learned a few things at the time.
We bought a Sonare which seems to be Powell's budget brand - it included a Powell signature headjoint and a B footjoint. We got it new for around $2200.00 Australian at the time (around 2002). Powell are a well renowned American flute manufacturer.
Now, some explanations:
The footjoint defines the lowest note the flute can produce. A "standard" flute will have a C footjoint, giving you a middle C as the lowest note you can play. A B foot joint takes you down to the B below middle C. Pretty simple really. It's just longer and has an extra key.
Open or closed holes: Advanced players use open hole flutes. This simply means the the closure of the holes depends on the pad of the finger rather than just the pad of the key. The key still has a pad, but the centre of the key is open which require's the players finger to block it. This requires accurate finger placement and some practice to get right. Many open hole flutes come with plastic inserts to block the holes while you learn how do do this correctly. I don't know if there is any real advantage other than possibly forcing better technique. Although thinking about it, it would be possible to change the pitch of a note (maybe harmonics) by closing the key but controlling the closure of the hole. I don't really know enough about flute technique to give definitive reasons.
The headjoint contains the mouthpiece. The orifice in the mouthpiece may be round or squared off. Near as I can tell, shape selection is purely a matter of player preference - and possibly reflects the players embouchure development. The material it is made of will have a great impact on the tonality of the instrument.
From the little I know (based on my wife's experiences), I would select a B footjoint and open holes BUT with the plastic inserts so I could still play whilst learning good technique.
Brand selection is not something I can really make recommendations about. My wife's student flute was a Buescher and quite good. She outgrew it and hence the Sonare.
However, like any instrument buy the best you can afford. This is an investment in your musical future and if you get something that is poor quality you will quickly become frustrated with its limitations and think that it's all too hard for you and that you'll never get it when in reality its the instrument letting you down.
Yamaha student instruments do have a good reputation, but they clearly are not up to the standard of their professional instruments. At least this is true of their brass - I assume its the same across their range. I have no real knowledge of Armstrong.
What I would probably do is look for a second hand professional grade flute in good condition rather than a new student model. This way it will continue to be a good instrument while you develop as a player and won't hamper you with its limitations.
Try to find a good instrument repairer and ask their advice RE brand (and model in some cases). They know the good ones and don't have a vested interest in the sale.
I notice Mac and Matt posted while I've been interrogating my wife
and preparing this little missive. Matt's experience is different to my wife's. She was constantly frustrated with the lack of that low B on her Buescher so I guess it depends on the music you play whether the B foot is a necessity or a luxury.