Runcam 2 system
The first first person view (FPV) system I used was a Runcam 2 system.
The advantage was that is both fitted under my drone, as well as in a LEGO train (allowing you to get a driver seat view).
The disadvantage was that the runcam behaves as a wifi basestation, to which you connect the iPad or iPhone. This was not a stable connection. In addition, it interfered with the control of the drone, also at 2.5 GHz. Since I wanted to always be able to control my drone, I turned wifi off, and only made recordings on the flash card. The second disadvantage was that I was frequently unable to turn the camera on: pressing the on button simply did not turn it on. I bought a second battery, but the problem persisted. After about it year, I gave up using the Runcam 2.
5.8 GHz system
Instead of wifi, I bought a camera and receiver based on 5.8 GHz transmission. This system simply transmits analog video streams, and is frequently use for racing drones. That advantage is there there is virtually no delay between the time of recording and the display on screen. In addition, when the connection becomes too weak, you will get more noise, but the video feed may still be eligible. This is unlike a wifi connection which has longer latency and drops the connection if there is too much noise.
The camera is a Hawkeye Firefly Fortress Micro FPV Camera.
The receiver is an unbranded "5.8G FPV UAV video wireless transmission". It features a 4.3 inch screen, and build-in battery.
battery: 600 mA, 3.7 Volt