I started off with a bunch of RPi's B ver 2 as you. Now, for the time being, they are all stored away, awaiting an upcoming trip to our recycle station. Today, I have only RPi3's running, I think 7 in all in our "production system", i.e. home automation & video monitoring & vpn server to give access from outside
For development & testing I have some more RPi3's mounted in a rack to make them easy accessible etc
The RPi3 is a noticeable upgrade from RPi2 in terms of performance. And it's nice with the WiFi and BT integrated. But as one of my old sw developer once stated: "a computer can never be too fast"
It also depends heavily on what you like to do. When handling real time critical video analytics with OpenCV or presenting heavy charts with many data points in the dashboard, well then it's getting tough also for the RPi3, you start to hope for the next, like RPi5 or so
So instead, for heavy charts, I store and save the collected data in a Windows PC (Gigabyte BRIX with a SSD disk) using sqlite3 as database, then using Highchart for making nice charts
I also looked at ODROID XU4 but so far I did not buy one. It looks to me as it is around 5-6 times faster than the RPi3 but at a 3 times the price.
Other thoughts I had was to upgrade a retired laptop with Linux but it did not have a Nvidia GPU with CUDA support, a requirement if you want OpenCV to run much faster