At Hi Sushi, the restraint is that you can eat as much as you want within reason, that is, you get a huge quantity of everything, more than you could want, and are allowed to make "orders" on top of that. This detracts from the buffet-style quality that Yo Sushi had, but on the upside, the quality of the sushi supplied is noticeably better:
It was a while ago that I went, so unfortunately my memory of the actual names of items is a little hazy. However, in the photo above, I loved everything served other than the brown thing on the left.
In terms of the quality/quantity trade-off, I would say that this place has the beating of Yo Sushi. Prices are about similar in the two, but in Yo Sushi they're fairly brazen about the fact that for £19.50, you won't be tasting any of their premium stuff. Pretty much everything I had to eat in Hi Sushi was premium, carefully crafted and satiated by taste buds wonderfully. Thus, it is very probably that I'll be saying Hi to Hi Sushi before I re-say Yo to Yo Sush.