yeah I started gym about 3 years ago and I love it I am the most inflexible person alive who does gymnastics i think and I am the Victorian champion AA i don't think felixibility is the most important thing I think it is that you are dedicated to the sport and love it then you can accomplish anything anyway you said that you were pretty flexible so there is nothing to worry about. Beam is not scary at all I think that everyone who starts gymnastics at first is scared of beam but then you will over come the feat and really enjoy it :) my favorite apparatus is beam ;)
Gymnastics I just started and love it<3
Trampoline is easier. If you notice, many trampolinists are in their middle age. Don't worry about falling from the trampoline because in a competition there are staff carrying a mat in standby to cushion your fall.
I would say neither because both need dedication. Also gymnastics you need to be extremely flexible and you will find that will come very hard to accomplish.
With trampoline, I am afraid I will fall and get seriously injured...does this happen often to people who do trampoline? And with gymnastics, I know you can get seriously injured....and I am a little afraid of doing the beam:/