Well they are proven to last very long so quality is very good.
They also sound very good.
Some people think that they are too small over the ears but most DJs like them for the light weight and good sound.
Differences as you can see below is that HD 25-SP II does not have the same frequence responce making them sound less good. IMO there is a very big difference in sound and I do not recommend the cheaper alternative as they do not sound good to me.
http://www.sennheisernordic.com/nordic/home_se.nsf/root/private_headphones_dj-headphones_502103
Says:
The closed-back HD 25-1 IIs are purpose-designed, professional monitoring headphones offering high attenuation of background noise. Capable of handling very high sound pressure levels and of extremely robust construction, these headphones perform exceptionally well in high-noise environments, e.g. ENG, sound reinforcement, studio monitoring and audio equipment testing. The capsule is rotatable for one-ear listening. Ideal monitoring headphones for cameramen and DJs.
Ear coupling supraaural
Transducer principle dynamic, closed
Nominal impedance 70 Ω
Frequency response (headphones) 16 – 22000 Hz
Max. Sound pressure level (aktiv) 120 dB
Load rating 200 mW
THD, total harmonic distortion < 0,3 %
Contact pressure ca. 2.5 N
Jack plug 3,5/6,3 mm stereo
Cable length 1,5 m
Weight w/o cable ca. 140 g
The HD 25-SP II are closed dynamic headphones for monitoring, recording and outdoor applications. They are a lower-cost alternative to the HD 25 II, featuring a simpler headband and a slightly different capsule design.
Ear coupling 60 Ohm
Transducer principle dynamic, closed
Nominal impedance 60 Ω
Frequency response (headphones) 30 – 16000 Hz
Max. Sound pressure level (aktiv) 114 dB
Load rating 200 mW stereo
THD, total harmonic distortion <0,4 % dynamic, closed
Contact pressure ca. 3 N supraaural
Jack plug 3,5/6,3 mm stereo
Cable length 3 m
Weight w/o cable 115 g
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