if you are going to be using your computer for providing video to your television.... then an external soundcard would be what you want. the other device is more for non-pc hookups (from the tv, from a dvd player, etcetera). you were not very specific in your question.
how will it sound? thats a hard call. no-name brands can be questionable but you have a 50/50 chance of it being acceptable. if you can look to see if it gets 4 or 5 star reviews on amazon or elsewhere if so it may be worth a chance, if not its likely junk.
while i understand you wouldnt want to spend much money on a soundcard for only having $100 speakers... the products from asus and creative are well known and respected and the externals are rather limited in models...