I only have the glass ones that come in the red box. I like them and now I'm waiting for the Joyballs de Luxe (which are a string type) so I can't really compare them. The weighted ones do look tempting too.
The price could be different manufacturers. The ones that I have are a lot heavier than the ones with the hearts if you look at the description.
I haven't had any problems with them being loose and just sort of falling out. They definitely do make a difference in your muscles and, therefore, your orgasms. If you're really concerned I would think that the lighter ones would be a better place to start.