Well, I'm a figure skating coach so I'm a bit biased, but I am actually going to say either/or. They are actually both really different. I've been ice skating competitively for 12 years now, and I can hardly stand up on rollerblades. It is such a different feeling. There are pros and cons to starting with either one.
If they ice skate first, they will get used to standing and moving on a thin blade (DO NOT use double runners), and then the wider rollerblade wheel will seem easier to balance on if they learn that later. On the other hand, if they rollerblade first, they will be used to pushing off the sides of their feet, rather than pushing with the toe pick like so many beginning skaters do.
So really, they are just different. Rollerblading can be done anywhere so that can be easier, with ice skating, you have to find a rink, and hope that they are open hours that are convenient for you.
Ice skating has more possibilities for future shows and competitions if that is the route they ever want to go. Rollerblading would too I guess, speedskating or...freestyling like they do at the X-games.
Okay, so I probably was no help at all. But those are at least some factors to consider.
Feel free to PM me if you have any questions at all about ice skating!