See my blog in which I compare and contrast Impetus in 15mm and 25mm and fail to come to any conclusion :D
http://leadwarriordavek.blogspot.co.uk/2012/07/impetus-25mm-or-15mm.html
Hi Dave
We have played Impetus in 15mm and 28mm as well, but we default to 15mm so our experience has been the inverse of yours. We habitually set up with 15mms 'close enough' to get a battle in, and discover with 28mm that warband become invincible because you're too close and cannot get out of their way.
In 15mm you do need to remember to set up closer, and with 28mm some people have a table that is too shallow.
Jim