Data traffic count reliability

[riskz]

Hi guys, I'm new here. I recently got a Defy and I'm beggining to learn to use Android. I got a limited data plan so here is the question:
Can I rely on Onavo/Watchdog data count? The stock counter under settings shows a lot more usage (I suppose it's because it counts wifi too, but I'm not sure).

Thanks in advance (and sorry for my english)