Call your vet. You'll get information and it will help establish a relationship with him or her.
There are a few theories on deworming.
1.) Rotating de-wormers. Basically de-worming every other month with different types of de-wormers. However, more recent research is showing this may be contributing to resistance in the parasites.
2.) Having a fecal done. The vet analyzes some poo and tells you what's present and you pick a de-worming product based on that.
If looking at switching de-wormers that have the same product, look at ingredients, amounts, and purpose. (Actually, that can apply for any product.)
COTH may actually be a better venue to ask. You can get a lot of information and opinions. I'm not saying HMF doesn't know anything, I'm saying COTH is more active currently and there are many professionals located there.http://www.chronofhorse.com/forum/index.php
I don't know how much experience you have with horses or horse ownership, but do you have a trainer? Do you board?