Grey market or not, it doesn't matter at all, because every product is genuine.
Warranty is not an issue as all manufactures offer just 1 year warranty which is practically useless.
Still if you want to make a choice in 10k, i'd say go with 590IS, though its not exactly pocketable but its somewhat very portable.
Other than that 870IS will only suffice if you buy from a Grey source so that what you get is VFM.
Just my 2 cents.