The core plan is plain ol' clean eatin'.
If you're going to do WW I'd recommend the core plan since I consider it a lifestyle change rather than a temporary diet.
If you do the flex plan you can eat as much crap as your points allow and to me that's just a diet. I would think that once you hit your goal weight you'd either stop dieting and maybe gain the weight back or you'd have to count points for the rest of your life.
Can you tell I tried the flex plan for about 1 week before I lost my mind figuring out points? LOL! To me it's ridiculous! After my DD was born I lost the extra baby weight through clean eating w/portion control and exercise. I lost the weight a long,long time ago but I'm still eating clean and running. So, I'm not experiencing weight loss but am instead monitoring my body composition. It's an ongoing process and my scale hardly fits into the equation. I think too much emphasis is placed on body weight rather than body fat percentages.
Bottom line, TRY THE CORE PLAN!!! I think you'll love it if a healthy lifestyle is your goal.