The meaning of both the sentences are similar. But in the language of Mathematics, the first sentence should be prefered than the second one. And don't forget to use Coma (,) whenever you use 'IF'. So, your sentence is "If you add 3 to 5, you'll get 8."

They do mean the same thing. In fact, you could also say "If you add 3 and 5 you get 8".