Microstepping - Constant freq. vs constant off-time.

Hi all,
Here's a question for the electronic guys.
Looking at some datsheets for stepper controller IC's i read that it's
better to use a constant off-time chopper instead of a constant frequency
chopper in a microstepping drive.
What is the difference and what difference does it make? Would it be a bad
idea to use the L6506 in a microstepping circuit just because it's a
constant
frequency controller?

Thanks!
/Henrik Olsson. Sweden.

PS. For those of you interested in stepper driving circuits I can recomend
the ST app-notes and datasheet site at: http://www.st.com/stonline/books/