External battery pack for weddings

I am looking into buying 2 external battery packs for wedding photography. The original Canon ones are expensive. The aftermarket ones are considerably less expensive. Should I spend more on the expensive Canon one or it is fine to get the cheaper aftermarket ones?
Thanks for the advice!

I have the original Canon external battery pack which was mega expensive. If I needed to get another one I would buy one from FlashZebra. These guys make great accessories of very good quality and price.Great service and prompt shipping too.
http://www.flashzebra.com/products/0155/index.shtml
That is the link to the Canon external pack they make. Looks exactly like Canons and half the price. I would guess the quality and performance of it is as good as the other stuff I have ordered from them in the past.

I ended up buying a cheap one from ebay. It was a mistake. The battery door broke during the first wedding. It is totally useless... Get the flashzebra or the original Canon!