Personally, it is whichever one I think will work nicer. I hate messing with fractions or decimals, so if I can keep them as whole numbers, I will. For example, which version would you rather substitute?
[tex]y = 3x + 5\\OR\\x = \frac{y}{3} -\frac{5}{3}[/tex]
These are the exact same thing, but it is almost always better to go with the first one. It's easier to manipulate and most people have an easier time working with whole numbers over fractions any day of the week.
