Sometimes I have only one sushi rice and want to prepare risotto, sometimes is the other way around.
Both varieties have a lot of starch, but I don't know if it's the same starch. Is it?
I've done risotto with other short grain rices (which includes sushi rice), with good results. I've even used medium grain rice in a pinch, and you'll have to stir more, but it'll come out.
I'd assume the same would be true in reverse -- that most short grain rices (incl. arborio) would make a decent sushi ... unless it's short grained brown rice, that'd likely less you up.
Basically, there's three main varieties of rice: short, medium and long (not including 'wild rice' or how it's treated (white vs. brown vs. parboiled). So long as you stay within one group, you're usually fine, so long as you don't change the treatment or technique used to cook it.
According to a Cook's Illustrated test, other medium and short grain rices (including sushi rice) can be used to make a creamy risotto, though it may not turn out "al dente" like with arborio.
Because of the "chalk" in arborio, it might be too firm for good sushi, but I've never tried it.
From a purist point of view, and ask my partner, apparently I hold a few of them:-) the best sushi will be made with sushi rice. But if you fancy sushi and it's teaming down with rain, like it is tonight,the store cupboard has arborio or a wet dark trip to the Japanese store the other side of town. I'd make it with arborio and it would be good. Same the other way. They are acceptable substitutes IMHO. On the other hand if you are entertaining your boss and are in line for a promotion. Get on your bike and make the trip. It will be worth it and you will have a story to tell