There's two ways you can look at this:
1) Slower weapons hit harder, so when Reckoning is up you get some nice big hits back to back. Thing is you don't use all your charges, so some of Reckoning is 'wasted'.
2) With a faster weapon you will use all 4 Reckoning charges, but the damage done each time will be much less.
I don't know the math, and personally don't care as far as fast vs slow weapon damage with Reckoning up. I like to keep my weapon speed right around 1.8 seconds: makes use of all 4 Reckoning charges but is still slow enough to do decent damage.
Something like this would be my choice if I was looking for a good Reckoning-worth weapon for that level:
http://wow.allakhazam.com/db/item.html?source=live;witem=9392
EDIT: typo.
Edited, May 6th 2007 7:31am by Maulgak