My thinking was that there would be less languages than IPs and IPs are those of the ISP not necessarily the user's country.
As for easier, scratch that. If there's a plugin all ready to drop in, it can't get much easier than that.
I was later wondering if the language code would necessarily reflect the user's country accurately.
So I guess the best would be to use the IP based plugin and if it doesn't initially show the user's currency provide a way to switch to it, which the plugin does.
Allows visitors to change their currency via a selection box
So if it works you should be good to go