WH2O has a way bigger variety of shots to make. There won't be another game like it ever made. The music is pure genius as you advance.
MM has the simple fan layout. It is a great game for non-pinheads because it's easy to destroy a castle and they get that great feeling of accomplishment. Getting "the holy grail" of pins and getting it NIB is going to be a one of a kind experience that needs to be strongly considered.
If you are choosing on price alone, obviously you could get two great games for one MMr, so your choice really has to be for the love of MM.