What do you need to do on your travels? They are very different tools.
The iPad is convenient, much easier to use on planes, trains etc, and has retina screen. I find with a cellular model not relying on public wifi, I can do everything I need to do when mobile with accessories such as a case keyboard or a very light keyboard such as Logitech's Keys To Go, and a stylus.
The MacBook Air doesn't have a retina screen or 3G, but would be a better choice if you need computer power that an iPad can't do at all or doesn't do as well.