Wheel size matters. If you use your hoverboard mostly indoors, any of them will suffice, but if you are outdoors, think about how many cracks or bumps you will encounter. The 6.5 inch hub will handle up to quarter inch bumps pretty well, but if they are frequent or larger than that, the larger 8 inch wheels help.
If you are using your hoverboard to go larger distances, a 10 inch hub will be even more comfortable yet. So this decision becomes a balance between comfort and portability. For most users, we think the 8 inch wheels are just about perfect.