duh-svee-da-nee-ya = до свидания - standard (good form) pronunciation
duh-svee-da-nya = до свиданья - more relaxed and coloquial pronunciation

Both are used equally often and you may choose whichever you like. The second won't make you sound uneducated - it's perfectly fine.

You can hear both variants of pronunciation in this song - the singer (Alla Pugachova) repeats in refrain "До свиданья, лето, до свидания" (Goodbye, summer, goodbye), i.e. she uses a colloquial form first, and then a standard one (eg. 0:29).