im still learning japanese but, i think the diference is おいしいです is more correct than the other, it still the same but the first one is more afirmative, like "it's delicious" and the second one is more like "delicious". that hapens too with 日本人です and 日本人だわ.like in "doudatta, hajimete no tokyo wa" its a non correct way to say id but is very common.hope this can help you Ps:sorry i didnt write on kanji and hiragana, this keyboard doesnt allow me to do it.and...im still learning english so please forgive my gramatical mistakes.