Should you get oblivion GOTY or collectors ed?

Depends which you prefer. Both have the full Oblivion game.

The GOTY edition comes with the two official expansions, Knights of the Nine and Shivering Isles, which add new items and places. You also get a paper map.

The Collector's edition comes with some collector items - a coin, a guide to the continent of Tamriel, a Behind the Scenes DVD.

If the collector items appeal to you most, then get the Collector's edition, and then pick up Shivering Isles at the least, you should be able to find it cheap.

If you don't care about the collectables, then go for the GOTY edition.