For the second sentence, since ‘nikui’ is an i-adjective, you can’t have the declarative ‘da’ at the end, unless you’re using the explaining ‘no’ particle. It would be correct to say ‘tabenikui noda’ or ‘tabenikui nda’.
For the first sentence, if you’re trying to say ‘eating sushi is difficult’, the nominalizing ‘no’ particle should be used. It might sound better if you say ‘sushi wo taberuno ha muzukashii desu.’
Both of these types of sentences are grammatically equally polite. It just depends on whether you have ‘desu’ on the end or not.
Sorry I have to write in romaji! I’m not at my home computer right now. ^^