Two different usages. The A/F meter will help you monitor the mixture during your actual driving conditions but should NOT be a substitute for keeping an eye on your plugs. (The gauge can be saying you're perfect but you wonder why it feels sluggish!) The dyno is good for a one-time tune and readout of the output of the engine, so if you have the money why not do both. Dyno time is obviously much more expensive, but keep in mind you'll need to poke a hole in the exhaust and weld in a threaded bung for the O2 sensor(all of which should be included with the gauge kit). I personally don't trust the wiz-bang clamp-on crap. I too have had good results with AEM but your experience may differ..
Some things to keep in mind once/if you get one--As others have said, don't get hung up on the gauge number. Most of our stuff often isn't happy or making the most power at the '14.7' reading. It takes tuning work to get it nailed at idle/cruise/WOT, then see what the gauge says when the plugs and performance are right, and tinker periodically to keep the number in that neighborhood.