Well let me put it in this perspective for you.
If you have the money and you don't mind spending it, then buy it. Reason: if you like the lens, you can keep it. If you don't like it, you can sell it off. Whereas if you rent, if you like it, you can't keep it and still have to buy it.
A word of caution though: This method works better if you want to try it over a week or a month before deciding. If you really just intend to shoot for a birthday party and then decide, then maybe renting would be a less costlier option but this is likely a lens you'll enjoy using.
Hope this helps. Sorry I seem to be sitting on the fence on this one. haha