This is very much a personal preference. Here is some useful information about each option:
The hollow ring is a slightly thicker (25mm) stainless steel bar, making the apparatus somewhat less painful to use. It is light, ideal for traveling, however it can be difficult to use for some drops, as it will ‘fling’ away from you. Hollow rings also don’t keep their spin for long.
Solid rings are a thinner bar (22mm) stainless steel bar, therefore can be a little bit more painful to use. It is preferred by most professional performers as it keeps its spin much longer.