Go to COLLABORATE, definitely.
If you read the blog I kept during OpenWorld 2007, you might have been able that I was not a fan of OpenWorld. It was very, very marketing-oriented. 80% of the presentations in the Hyperion track were delivered by Oracle (in some cases, with clients/partners as co-speakers). COLLABORATE is supposed to have 100-150 presentations with 100+ of those delivered by clients and partners.
In the interest of full-disclosure, my company is paying to be a 4-star partner of COLLABORATE, because we're hoping that it becomes the successor to the dearly departed Hyperion Solutions conference. While I haven't been to COLLABORATE before, I'm hard-pressed to believe that COLLABORATE could be worse than OpenWorld...
Hope that helps,