Well, the only way to get a legit "night and day difference" would be to have content that supports 9.1.

Let me go ahead and tell you, you probably won't find any.

Now, thanks to a little thing called matrixing, you don't need 9.1 content for 9.1 to just work, but if you take 5.1 content and play it through a 9.1 system, you're just not going to get that much of a difference unless you're trying to fill a huge space where there's alot of gaps in your sound to begin with.

With that said, whether or not it's worth it honestly depends. First things first, if you've got the kind of space where 9 speakers is honestly worth bothering with, you've got the kind of space where you also need two subs. To summarize what I'm getting at here, 7.2 should happen long before 9.1 is even considered. Outside of the fact that seven channel content actually exists in normal consumer settings, sound needs to be balanced if you're going to look into taking things this far. So assuming the nine speakers is something that's more "necessary" than excessive, you should probably look into getting a second sub first.

Secondly, realize that next to no content is actually going to support this, so you're looking at limited use from at least two speakers and potentially up to four speakers depending on the content. Even when in use, if you don't set the "extras" up properly, you might just do more harm than good to your sound. Since you're asking this kind of question here of all places, I'm guessing you might not really get into the fine details of setting up sound systems, but if you're going to go for nine channels you definitely better make sure you do it right or things are going to get strange when you're listening to content.

As for Netflix, it does support 7.1. I'm not sure whether or not any of the actual content on Netflix supports 7.1, but in theory it is indeed a possibility.

TL;DR -> You'll get a whole hell of alot more mileage out of going 7.2 than going 9.1 right now and in the foreseeable future. Unless Dolby Atmos or DTS: X actually take off (which will be where you see content actually supporting 9+ channels), 7.1 is going to become the next gold standard.