BT rocks the house when you have a built in module in your laptop! i have a had both 'wireless' and BT. with the wireless you have to have an external device that plugs into your USB. with bluetooth, since you have it built in, all you need is the mouse. great for portability purposes. also better in a technical sense. the RF (radio frequency) range of a 'wireless' mouse is the same as your 'wireless' phone at home. and can get interferance relativly easy. the RF of the BT is a different range so you are less likely to get interfereance, and BT has been designed to allow for a longer range.
i would recomend The Mouse BT from macmice.com it looks like the apple mouse, except it has two buttons and a scroll wheel. as you mentioned it, like other BT mice are on the expensive side, but i deffinitly think it is worht the extra money! especially cause it will look sooo good sitting next to your iBook!