The purpose of both Power Grips and clipless pedals is to provide foot retention, which many roadcyclists find useful. Power Grips are thick diagonal straps that bind your feet to the pedals while still allowing easy escape. They can be worn with street shoes. To extract your foot, you must slide it back while rotating the heel outward. Clipless pedals contain binding mechanisms that latch on to the cleats attached to the bottom of cycling shoes. They can only be worn with special shoes. To extract your foot, you must rotate the heel outward until the cleat disengages from the pedal.
As far as foot retention, I do not feel a huge benefit to using clipless over Power Grips (assuming that the Grips are adjusted tightly). In two years of using the latter, never once have my feet accidentally come out. With the clipless setup, the connection to the pedals is more secure and more precise, but in all honesty I am not sure it's necessarily "better" as far as power transfer and what not, at least not for someone at my current level of ability. I have not noticed any difference in speed after switching to clipless.
The ease of clipping in and unclipping feels about the same. Having found a brand and model of clipless pedals I am comfortable with, I do not experience difficulty using them. Clipping in does require more precision than sliding my foot inside a Power Grip, but once mastered it is actually faster. Clipping out is very similar to extracting my foot from a Grip, requiring the same rotation of the heel. The time it takes me to unclip is about the same.
The main reason I prefer my new clipless setup to Power Grips - and this was entirely unexpected - is that I find it considerably more comfortable. I had not even realised how much discomfort I was experiencing until I switched to the clipless shoes. Before I would occasionally get blisters and numbness from the straps, which is no longer an issue. And it's great not to feel anything pressing down on top of my feet. I do not feel the spots where my shoes attach to the pedals, so there is no pressure on the soles of my feet either.
Needless to say, these are all just my personal impressions and are by no means intended as some definitive verdict. I've been getting requests for a comparison and did not want to wait until the memory of using Power Grips faded. As before, I think Power Grips are a fantastically useful product for those who do not feel comfortable with clipless pedals or simply do not want the commitment of having to wear special cycling shoes on their roadbike. I have done paceline rides and fast group rides with Power Grips, and it's been fine. That said, the difference in comfort I've experienced makes me glad I've finally mastered clipless, and I don't see myself going back.