I have SPDs on my MTB and Looks on my roadbike. Same situation you're referring to. I wouldn't worry. The motions of clipping in and out at the pretty much the same. If anything it'll take a few rides to get used to flipping the Looks over since you'll be used to double sided entry on the SPDs but it's not a big deal.
The Shimano and Time pedals are similar to the Looks, but I have not tried those. I would like to try some Speedplays to see what the deal is. I have heard both good things and bad, I guess it could be a matter of personal preference.