I'd argue that erasing the division between front and back end developers will make the question of "which is harder" irrelevant. It may slow you down at first, as people get up to speed with the parts they're not familiar with.

I'm of the opinion that by far the best way to accomplish this is by pairing. Start with asking them to help you on something you'd normally work on. Show them how a failing test makes the job easier. Once you've got some buy in, try following the feature through the front end, all the while using tests to "prove to yourself" what you're doing is working.

There a nice video from Velocity on creating cultural change that's worth watching, even if not directly relevant. If at all possible get people to join you, don't tell them what they should be doing.