That's an impossible question for us to answer, because it totally depends on how many expanded voices you intend to install. The sizes of packs varies enormously from a few MB to hundreds of MB.
When you buy a flash expansion, if you then decide it's not enough, you have to throw it away and buy a whole new one - you can't add to it. So that's an argument to buy the largest flash you can.
But if you then find you're not that impressed with the expansion voices, you have spent more than you needed to.
It's really your choice. Take into account if you are considering upgrading Tyros to Genos in the near future, because it's unlikely a trade in would give you anything like the value you paid for the flash card.