Nintex is extremely expensive... They can be since there are few alternatives. I used flow for some workflows and it seemed to share a burden of many other apps of MS: Good idea but not really finished for productive application. There is nearly no depth to flow-workflows for more complex operations. On the other hand, there might be integration for other services your org is using.
PowerApps also seems to be a very interesting Application, since you can create apps for other platforms that can use your SP lists as a basis for data. A form as a native app for many devices. Last time I used it it was still extremely slow and unstable. Desktop and Browser Versions. And you don't have advanced configuration options if you just need a little bit of extra logic.
I would suggest you just try Flow/PowerApps yourself. They are both relatively simple applications. In my opinion, they still don't even have the feature set of classic SP-Workflows(2010/2013), not even speaking of Nintex. But they might work for you in combination to save some money.