The 50cm (fatter) are the latest generation of USB Risers. The main difference is that it has 3 caps for power filtering while the older generation 60cm (skinny) has 2 caps for power filtering. Honestly, either are fine, I actually prefer the skinner versions as they look a little neater. You can use these on any PCI-E slot, so 16x would work.

As for the difference in quality, I cannot comment. Most likely they're the same since these are only manufactured by a few companies in China right now. However, I can comment that 40% of our risers are QC at the manufacture level and I personally QC 10% of riser batches.

Cheers,
John