Welcome to the wonderful world of airsoft mate!
Just had a look at them both for you. As others have said above, Internally they're pretty much the same, with the same features (Rotary hop, 6.03 tight bore inner barrel etc). The main difference between the 2 is that the F-02 FLEX is longer with a longer inner barrel whereas the CORE is shorter and more compact with a shorter inner barrel.
Which ever you decide to go for will depend on your play style and which you prefer the look of, they should both perform pretty much the same.
Ask yourself, is your site more CQB based or longer range outdoor? do you prefer getting balls deep in the action cutting tight corners whilst clearing small buildings, doorways and corridors? Do you prefer and feel more comfortable handling a longer or a shorter RIF? This will help you decide which one to get as the main and only difference i can spot is the length.
Hope this helps!