What most users and reviewers will tell you is that it depends on what you need most - great reverb or great limiters and compressors. Almost everyone agrees the Realverb on the UAD is sub-par, and the ClassicVerb on the Powercore is world class. At the same time, the vintage comp/limiters on the UAD are highly praised, with so so reviews of the powercore dynamics. For EQ, the holy grail is the Sony Oxford, which runs on Powercore, but is an expensive add.
Any idea just how expensive?
UAD has the Pulltec, a legendary program EQ, but it's not a great surgical EQ like the Sony.
Myself, I went with the Powercore first, intending to add a UAD later. I wanted great reverb now, and figured adding a second card would be easier if it was the less expensive of the two. Anyway, They are both great cards by most all accounts, with different strengths.