I'd get it, then stop off at the vets and ask them what the difference is if any. I'm betting there isn't.
I got stuck buying Heartgard at a vets once. My hubs mentioned we were running low, and since our usual vet just ok's the prescription on petmeds, hubs was wondering if she would do the same. Before he was finished talking, she asked how much he weighed, then went into a cabinet and got it. Next thing I know I'm paying almost twice as much as Petmeds charges..
They sell a lot of the items we can get elsewhere, and they sell it for much more. Drives me crazy.