Harder. A no-brainer. With positive foot retention there is no excuse for loafing and just letting the pedal pick your foot up. No, now you realize that you can actually lift that pedal so all of your downstroke is powering you, not simply pushing dead weight around. Fun! And the by-product - you go faster. More fun! And the result of all this fun? Well you did the same ride in less time, did more work with more muscles, burned more calories, are more spent and have more tired muscles. Yes, harder.
Now you can choose to simply let your fully restrained feet just get pushed up like before and change nothing except the need to twist your foot to dismount. Then, no: retention is not harder. And not more fun. For me, every level of more retention I graduated to meant going faster with a bigger grin on my face. First toeclips. Then pulling the straps tight. Wow! Then pulling those straps tight around cycling shoes with slotted cleats. Double wow! Clipless meant getting started was easier and I was allowed to forget to unbuckle but the wows stayed the same.