They are very different mics. (I do have both in my own mic. box).
The 421 is a
cardioid and can be used for a wide range of applications (including, brass, vocal,
speech, drums, etc.....). Everyone should have one, it is so versatile.
441 is a super-cardioid and Sennheiser designed it to be the closest thing to a condenser
mic. in a dynamic. It is a superb mic. (even good for vocals if you remember the old
Elton John poster).
It is nice to have both - if you only have enough money for
one I would go for the 421, if you don't have one already.
I don't know how
much you are being asked for the s/h ones but the brand-new price (ex-VAT) is £342 for
the 421 and £599 for the 441.
I hope this helps.
John - Sound-Link ProAudio
President - Federation Internationale des Chasseurs de Sons