no true gamer will ask this sort of question ..... they will just go out and buy it already >_<:

just recently i believe they had a WOL discount (was it 50% off ?)

although i am not sure if that promotion is still ongoing, or whether there is a promotion to combine the purchase of WOL with HOTS for a further discount (which i doubt)
So your best bet i would think is getting the WOL with the discount (if it's still available), and buy HOTS separately via digital purchase which is 39.99 usd via battlenet store if i am not mistaken.

There doesn't seem to be a reason to get the hardcopy, unless your going collector's edition which didn't really seem worth it to me.