I chose Flex because I didn't want to be told what I could & could not eat.
The thing that I don't think I'm ready to handle on Core is the satiation point. That's a difficult thing for me to think about. So I rely on portion sizes through the points to help me get in the amount of food I need.
I think that if you'd really like to try Core that you do it. No sense in getting frustrated if you don't try the plan that you like.
Flex is usually a good way to ease people into the program & into looking at portion sizes.
But, it's only a week that you have to try it out. So, whichever one you choose, you can always switch next week to try the other!