Users who choose between GNOME 2 and GNOME 3 are rarely making that decision on a purely rational basis. In my experience, users of GNOME 2 are often choosing what they know, while users of GNOME 3 are technophiles who enjoy anything that is new.
Neither is likely to go over the two generations of GNOME feature by feature. In many cases, the choice seems made before login.
I spent the day with the two desktop environments open side by side, looked at the desktop components in both, and tried to pick a winner in each basic category based on efficiency, ease of use, and the availability of choices for users.
The effort wasn't always easy. Neither being new nor being traditional was enough in many cases. Frankly, the switch from GNOME 2 to GNOME 3 often looked like an exchange of one set of shortcomings for another.
Still, the results highlight the strengths and weakness of both desktop environments -- even if you disagree with some of the conclusions.
So far as menus goes, choosing between GNOME versions is a matter of which inconveniences you prefer. Do you want your workspace blocked by submenus spilling across the desktop, as with GNOME 2's classical menu, or do you want to leave your workspace for a full-screen menu, as with GNOME 3?
One of GNOME 2's trademark is the Applications / Places / System set of top-level menus. The Places menu is of limited use, but the System menu has the advantage of making configuration and administration tools easy to find. In most implementations, it has no favorites menu, although you can easily create one using the Drawer applet or a launcher like the Avant Windows Navigator (AWN) on the desktop.
By contrast, GNOME 3's menu appears in the overview, not the main workspace. The overview installs with a launcher for favorites, and divides the menu into Windows and Applications. Configuration and administration tools are part of the general menu. However, you can use the System Tools filter to find them quickly.
In GNOME 3, reaching the menu requires two clicks: one to reach the overview, and a second to switch the menu from the default Windows to Applications. After that, both require two clicks, one on a sub-menu, and one on the application to launch anything. GNOME 3's filters are no advantage, since GNOME 2's sub-menus give much the same functionality.
Verdict: If not inconveniently placed on the overview, GNOME 3's menus would be the winner. After all, if using the menu is going to disturb your work anyway, the menu might as well give you a clear, full-screen view. As things are, both versions of GNOME have enough shortcomings that I have to declare a tie. However, I suspect that many users would prefer GNOME 2 just because it's more familiar.
Under GNOME 2, panels are an essential part of customizing the desktop. Besides basics such as the menu, notification tray, and clock, the panel was the place for applets, small utilities that could be added and positioned as suited you. About forty applets were available, about half of what KDE now offers, but enough to give users a choice.
The panel was so useful that many distributions shipped GNOME 2 with top and bottom panels, reserving the bottom one mainly for a windows list, so that it had enough room that users could easily keep track of applications that were minimized or running in the background.
GNOME 3 threw away the idea of the panel as an element for configuration. Only a few basic tools such as the clock and calendar and the system sound indicator remain. The panel still does duty as a window list, but shows only the currently active window, and not other open windows.
As for applets, forget them. Such applets that survive, such as Tomboy Notes, are lumped into the general application menu, where they are much less handy. Nor can you reposition the panel or its contents. Even a useful panel feature like notifications is switched to the bottom of the overview, where it is less obtrusive -- but so much so that you can easily miss events on the system altogether.
Verdict: GNOME 2. While GNOME 3's panel is less cluttered, it is also vastly less useful. For many, the inability to add a second panel is also a major handicap.