The rice is the essence of sushi. Besides nowadays the word "sushi" design wrongly the dish with raw fish: in fact su-shi literally means "sour rice". You need to choose good quality rice, and cook him properly (see the recipe).
The story of sushi as we know it now begins in South Asia , around 400 BC. The rice was used as a conservative for the fish in wood jars. At this time, it was simply threw away when consuming the fish (sometimes 1 year after!). Then in Japan, in Edo era (in the 17th century), it was added vinegar to improve the conservatives proprieties of the rice, and some started to eat the rice with the fish. The sushi was born !
As it contains high levels of carbs, the sushi isn't a "dietetic" food as most of today diets recommands low levels of carbs. However, the rice is an aliment with a high satiety power. Combine it with some healthy fish and nori, and it gives you an equilibred and nutritive dish, low in calories. So even on a diet, eating sushi once in a while won't be a drama!
From the cheap sushi rice in a random asiatic store to the top grade grains of some sushi webstore, there is a variety of choice, according your tastes and budget. But in every case, you should always buy short grain rice.