Before stating my opinion, I would comment that for years I used a mediocre quality 10' mesh BUD on Ku. The quality of that dish slowly deteriorated over the years due to knocking ice off in the winter, but when it was about 8 or 9 years old, I got a 3' solid (Fortec) Ku dish. Even at that age, the 10' mesh was significantly better than the 3' solid dish with respect to resolution, and somewhat better with respect to signal strength and quality readings on digital. Now that the 10' dish is nearing 14 years old, it is still better with respect to resolution, but now the signal strength and quality readings have deteriorated even more, so that the 3' dish is now significantly better, and I now primarily use the BUD only for C band, unless I need to evaluate adjacent sat interferrence.
Basically, my opinion is that:
(1) Given equal quality, bigger is ALWAYS better.
(2) Unless the mesh BUD is more than 20 years old, mesh size isn't important, as even to Ku, a mesh dish will perform as if it's a solid within ~1%.
(3) All else being relatively equal, the accuracy of shape of the dish is what is most important.
(4) It is probably easier to manufacture a quality solid dish than a solid mesh dish, if for no other reason than most mesh dishes are made from flat panels which are bent only radially away from the center, but really don't get shaped properly width wise (if that makes sense). Intuitively it seems like a pre-formed sectional mesh dish could be better quality than one that has slide in panels, but that is just a feeling not based on anything.
(5) Provided that you have a good quality BUD and a sensitive signal meter, it's really no harder to align a BUD for Ku than it is to align for C-band. Yes, the tolerances for Ku tracking are tighter, but the process is pretty much the same. However aligning a poor quality BUD with a cheap meter can be quite a task.
Given all the above, I'm convinced that a good quality BUD will work better than a 3' solid dish on Ku. But as you increase the size of a solid dish above 3', you'll do as good or better than the BUD, and as you decrease the size of the BUD below 10', the BUD will lose it's advantage. A low quality old, cheap or damaged BUD will be inferior to a good 3'dish. In addition to performance though is the convenience factor of having the 2 bands on separate dishes.
I'd recommend having a separate 3' dish for Ku just for the convenience alone, but if you have a good quality BUD, it can work well on Ku.