We know this optic is not new, but it's still quite new for the customers who just got into the sex doll world, please allow us to address this question here.
Pros: Softer & More realistic
Make the doll little bit lighter
Cons: Slightly more vulnerable to tear, but the material won't tear up easily, please don't overthink the risk here.
Unlikely to tear up
Both are very good choices, if you want to have better experience when you're spending time with your doll, you should probably go with the hollow breasts option, if you use the doll mainly for photos, solid breasts option is highly recommended.