Is it better to just buy a inflatable waterslide or rent?

Depends on how often you intend to use it, if only a couple of times then rent, think about if you have the right storage for it when not in use and the quality of the product you can afford as rental models will be or are more likely to be a better quality commercial grade

You of course need to look at costs of both.  Then weigh them against how many times you think you'll use it.  Average out the cost of buying per use.  Factor in the luxury of being able to use it whenever you want.  For me, storage when not being used would be a big concern as well.  Only then can you decide what if best *for you*.