the 201 has a bit more bass to it... but they are both great.... you might need to shelf the 201 a bit for guitar. unless you want it THICK.
lots more people use the 421... its all a matter of choice, they are close enough that you will have to discover this for yourself.
personally i like the 201 more (of course i have more experience with it...). it is smaller and easier to place. it uses a standard clip. you can use the proximity on it to make a shitload of bass... and it is usually a bit cheaper.
although the 421 looks a hell of a lot nicer.