The 250 Ω
T90 have more voltage sensitivity due to their lower impedance; in other words, the T90 play louder than the T1 and usually work great from portables. (You'll need a
Grado Mini Adapter to plug the T1 into portables.)
I prefer the sound of the T90 over the T1. The T90 is more entertaining, while the T1 is more accurate. They are very close to each other.
Compared to the beyerdynamic T90
The
beyerdynamic T90, with its lower 250 Ω versus 600 Ω impedance, have more voltage sensitivity, which means they play louder. The T90 sound great plugged directly into portable devices, while with the T1 you'll often want an outboard amplifier for it to play loud enough.
The T90 have slightly more treble and slightly more bass, which is more fun than the slightly emphasized midrange of the T1. When I say slightly, I mean a dB or a fraction of a dB here or there; these beyerdynamics all sound very similar.
I also prefer the bass of the T90. The T1 sounds very slightly tubbier to me, while the T90 has just a little bit more and tighter bass on direct comparison — but it's very close between the two.
With a little more on top and bottom, the T90 is more fun for music enjoyment, while the T1 is more accurate for music creation.
For musical enjoyment, I prefer the slightly more entertaining sound of the
beyerdynamic T90. For creating music (mastering and mixing), these T1 on the other hand will ensure your work will sound great everywhere, as the T1 lack the very slight high end and bass emphasis of the T90.
For portable devices, get the
T90 instead so it will play loudly enough. If you use the T1 with an iPod or other 3.5mm output device, be sure to get a
Grado Mini Adapter so it will plug in.
Never cheat yourself out of the headphones you deserve. They will last you a very, very long time. If you want the T1, get them and you'll never look back. If you really must economize, the
T90,
DT990 and
DT880 are also superb choices. When I have them all here for direct comparisons the differences are small, and if I had only one, I'd not notice. They are all superb; beyerdynamic knows how great headphones should sound and all these models are similarly excellent. Your preference should be guided by the slight differences in tonal balance, not by price. Get what you want.