I think it really depends on what you’re wanting them for and what you want to achieve.
If you want them for training your pelvic floor and working the kegel muscles then you want to go as small and as heavy as you can. You might need to start with slightly larger, not so heavy ones until your muscles strengthen though.
If you want them for pleasure and foreplay then large enough to substantially feel and with a jiggle ball movement definitely wins out. These wont tone your muscles as you’re doing very little work to keep them in, but they feel great.
My favourite two sets of each of these types were both from Lovehoney but my small, free standing metal ones aren’t sold anymore. Essentially though they are very similar (I think) to this 50 shades set but just two solid orbs without the retrieval cord. Maybe slightly smaller but a little lighter in weight. These 50 shades ones will be a workout for fairly established muscles but they’re probably not high on the pleasure scale (at least to me). I’ve not worn my small, free standing ones outside my house.
The set I recommend most to people for fun though are these XL Jiggle Balls. They are not a kegel exercise, you do not use muscles to keep them in because they are huge!!! They jiggle delightfully, are way bigger than any vibrating egg I’ve tried and are probably my favourite toy for wearing on a night out because they leave me very wet and ready when I get home. My review of these can be read here if you’re interested.
I’m also going to throw a tag out to @FortySomethingWife who is the forum queen of ben wa ball knowledge and has way more sets than myself and will have information to share if she is about.