All right, it seems like the last option, the Message Tag system, works with the Email client of your choice, so you'd need Outlook or Firefox, etc., and you can set the Tag system to auto-flag and track your mail, so you don't have to worry about all that.
The only real reason to use an e-mail client is if you want the e-mail to download to your computer, without having to save e-mails manually. Currently, I leave all the mail I want to save on the Gmail server. I may change that, and start using client software; however I digress...
Of the other two systems, I like the first one, that deals with the pixel, and tracks the pixel (although they warn that it doesn't always track). I need to look at these some more.