Kind of a loaded question but let me see if I understand. You have a call to action button and you want to use it in multiple places on your website but don't want a duplicate content penalty. You wonder if making the button in text might be more effective. Is that correct?
First of all, I think there is a lot of research out there that your image call to action will be more effective than any styled text could be. You have a lot of options with fonts, colors, shading, etc. in images, not so when styling text. I would definitely use a button in that regard. Also, you want it to stand out. That isn't always just bright colors, it's contrasting colors. SEOMoz for example has a light blue background. Right now there is a pink "call to action" of sorts (more of a look at me I'm important note) at the top for the expiring private messages. Stands out, doesn't it? Because a warm color (pink) contrasts with the cool color palette established by the blue background.
You don't have to worry about a duplicate content penalty on images that I'm aware of. What I would do is only use it once per page in the best place you can put it and make sure every time you use it on all your pages you maintain the same alt text. I've seen that it's possible Google isn't a fan of repeating an image with different alt text or calling lots of different images but using the same alt text.