I think clip-in on the Looks will be easier... give yourself a chance to get used to them. Also, it never hurts to practice clipping in and out a little. (Don't get me started on clipping in to my SPD-Rs. They're great, once you're in.)

As far as being harder to pedal, keep in mind that bike setup is the combination of a gazillion small and not so small adjustments and differences. The Looks have probably changed the distance between the bottom of your foot and the pedal axle (raise or lower your seat a little). Your foot may also be positioned differently relative to the axle (in front or behind), again changing your leg extension. Make it the same as your old clip-ins, or try shifting your seat forward/backwards a tad. Tiny differences feel major.

If it feels like there's more effort, I'd guess the pedals have probably slightly reduced your seat to pedal distance. Try adjusting your seat or cleat position to compensate. I'd suggest trying them out for a couple of weeks. If you did fine with less float, the black cleats are probably a good idea. If you get knee pain, something is seriously wrong. You might want to consult with someone at your local shop for suggestions... just don't ignore it.