What's the difference between the cheaper diverters and more expensive ones?

You're much better off investing in one of high end diverters as they're far more effective at filtering and diverting water and are much easier to maintain. With some more basic diverters, you may have to dismantle the downpipe to clean it, but with the Gutter Mate, for instance, you can empty the filter basket in 30 seconds or less without any dismantling needed.