Uhmmm well it depends on what context you're using them. When you say something is sweet then you're often referring to it's taste with respect to how much sugar is in the food.

When you say a food is delicious then you're basically saying that it tastes great, irrespective of whether the food is sweet, spicy etc.

But then again, I'm Nigerian and I know that in Nigeria when you say something is sweet then you're probably trying to say that it tastes good, irrespective of whether the food is sugary or not. So in a sense, the word 'sweet' in Nigeria is synonymous to 'delicious'.

Yes I know, Nigerians are awesome like that!!!!