The System-Profiler only shows, whether an application can be launched natively or as interpreted via Rosetta. On Intel machines running OSX you can choose, if you want to start an application natively or in "Rosetta-mode". This works for applications, whether they are running or not, if they are "Universal binaries". Not all are.
If an application is running, 'Activity Monitor' can look into the running application, and really "sees" the mode, as does RosettaTest.
The only significant difference is that the icon just pops into your eye, especially if your panel is floating, and you may avoid the overhead of 'Activity Monitor', watching disk usage, networking etc.
I used some applications with plugins and drivers. They were doing just fine, except the plugins. Sometimes even 'Safari" needed 'Rosetta-mode' in order to use additional functionality.

Just use activity monitor, have it show kind in the list and it will tell you if a process in intel or PPC.

If you need the information only once, then this advice is correct. If you want to check more often, while "Activity Monitor" just sits in the back, you have to grab the window and look for the correct line in order to get that information, besides the overload of informations, which comes not for free.
I love "Activity Monitor", but I am not running it all day long.