Open up Photos and plug in your SD Card or iDevice. Go to the "Import" tab and look at the upper left corner of the window.
All you need to do is uncheck the "Open Photos for this device" box and Photos will not autolaunch when you plug in your device.
The somewhat annoying part is that you will need to do this for each and every device you plug in. But once you have plugged them in and unchecked the box all should be well and Photos won't autolaunch.
Image Capture has been around a long time and is installed by default in the Applications folder. You can use Image Capture to do all sorts of things including using a scanner, sorting your photos, quick emails or even make a contact sheet. You can also set what you want to happen when you plug in a SD Card or idevice.
Find Image Capture in your applications folder and open it up. Once open look in the lower left corner of the window. You will see almost the same import configuration options as you see in Photos.
Using Image Capture also allows you to set the import location or what to do with the photos on import. Just use the "Import To" pull down menu at the bottom center of the window.
As you can see there are a lot of cool options, even build a web page! How easy is that.
If you are a photographer and take even a modest amount of photos, getting rid of small annoyances, saving time and streamlining your work can really pay off. These tips may not make you a better photographer but they definitely help your workflow.