Paper-based GTD planners have a section (marked by a tab) for Agendas. Each sheet of paper in that section is designated for a specific person or meeting. Write the name of the person or meeting at the top of the page and list the agenda items down the page as you collect them.
In an electronic GTD planner (e.g. MS Outlook) use the @Agendas category. Each "task" in that category represents a specific person or meeting. In the "Notes" section of each task, list the agenda items you want to discuss.
A paper-based tickler file is great for making sure that a piece of paper gets into your inbox and noticed at an appropriate time. For example, if I have concert tickets for a venue that's three weeks away I put it in my tickler file for the day before. When that day arrives the tickets are in my inbox, right on schedule. Paper bills and date-specific reminders scribbled on paper are also good candidates for this.
If you're having trouble deciding which to use then ask yourself which would put the information right in your face when you need it. Maybe consider doing both for a while. It's better to be a little redundant than let things slip through the cracks. As long as your brain trusts your system enough to let go of these reminders then you're fine.