What is RGH or JTAG?
  Both are means to exploiting the xbox360 and allowing the ability to run unsigned code, AKA homebrew apps. What it basically does is allow you to get root access to the xbox and run games off your hard drive and run emulators and xbla games all for free.

What is Flash
  Flash is a mod to your optical drive that allows you to play burned games. the down side to flash is that you can only play them off of dual layer discs that need to be burned from a specific disc drive. more information at junglefalsher.net the upside is that you can play on live and hopefully - though it is still possible - not get banned.

.::Recommendation::.
As for deciding what you want to do you need to decide what is best for you. Do you use xbox live a lot? If so you want to go with fw flashing. If you don't use xbox live a lot and want homebrew apps/emulators as well as XBLA games, DLC, Avatar items, and being able to run games from your hard drive, then RGH/jtag is a much better option for you.