Of the two, the Captivate. Much more modern OS, and there's no reason to tie yourself to BlackBerry Messenger if you're not already using it. The Bold 9780 is based on an old processor and an old OS. Just be aware: Samsung likes to abandon Android phones, so don't be surprised if you never get Android 2.3 or later.
A few tips of advice, though. First: what about the iPhone? It still arguably has the best touchscreen keyboard out there, and it has a better display than the Captivate (much sharper, not so oversaturated). There's the front camera, too; the Captivate doesn't have that. And Apple tends to support the OS much longer than most Android phone makers do, so you'll probably have updates for the next 1.5 to two years.
If you still want Android, there's also the Nexus S coming out by March and some newer phones that might be out at the same time. The Nexus S is very similar to the Captivate, but it has a front camera, some NFC wireless... and more importantly, the OS upgrades are controlled by Google, not by Samsung, so you can actually expect some long-term support. Rogers' roadmap for future Android phones isn't clear right now, but it should be easier to predict in February.