I spoke with some of the guys from my base about this vary topic. I am a member the Air Force 193d Special Operations Wing that specializes in Audio/Video transmission and receiving. We have lots experience with audio/video transmissions. Here is what they told me as a guideline to make basic decisions by...
-First, they said think of the chosen frequency like the frequency of the music in a car stereo when considering range and flying around objects. Like in the car stereo where low bass music seems to go through just about anything and go on and on, so does the lower video tx waves. The lower the freq, the more "blunt force" the waves seem to have at pushing further and further.
-Second, they said there is something else to consider, though, with regard to the size of the information going across the waves. The higher the frequency being transmitted, the more information seems to flow. This is in reference to higher definition video (which most people seem to strive for). Simply put, higher lines of video definition logically will transmit better on a higher tx frequency than a lower frequency.
So, it's a bit of a challenge for the beginner to determine what purchases to make the first time around. As for myself, I want the best definition I can get for my budget. However, I also want the range of the system to be as good as my Spektrum TX & Rcvr.
The other nice tidbit of information they gave me was that Transmitter strength (often measured in watts) is NOT directly proportional to range. For example, just because a 200mw system can effectively transmit one mile does not mean a 400mw system can transmit 2 miles. It will be less. They said their is a math formula to determine the effective range with regard to the power, but I wasn't about to work through that headache quite yet. For me to keep it simple for myself, I just say buy the strongest quality Tx that you can afford and works for your application.
Last, they have a device that is too cool for tuning an antenna. It looked like a large oscilloscope. They were showing me how they can first fine tune the circuits of the Tx and the Rcvr to be more fine tuned which will drastically increase their performance and quality of signal. Then they used this cool device to precisely measure the frequency the Tx is sending and then use the device to see how well the antenna is tuned. With that information, they could adjust the antenna down to a very precise fit for the frequency which helps get the best possible scenario. I love this stuff! (if only I knew how to do it myself!)
I'm not trying to jump in here sounding like I know everything about this topic, because I will be the first to say I am a newby, I just want to share a great resource that I believe we can use to help solve problems and answer questions!
Cheers~