We really love ON24. GoTo is great for giving tutorials, but we found it wasn't seamless for full-on presentations (ie: you can't get the cursor to disappear, polls would close out the PowerPoint presentation, etc). There was SOME work to do for Marketo integration, but nothing astronomical.
Also remember that GoTo doesn't currently let you use a Marketo email for confirmation. And when I chatted with BrightTalk, there was email customization, but not through Marketo (it seemed like the operational emails all went through BrightTalk, so you lose some tracking).
The price difference between GoTo and ON24 is a fairly large, but we felt it was worth it due to the ease-of-use and amazing customization options.
If you have any questions, let me know and I'll be glad to help!