I would go with a mix. Teach your students the basic concepts, and then let them try each software out. Explain to them the importance of not learning a tool, but rather a technique. Then let them decide which tool they prefer, and have seminars where the different user groups show each other how they accomplished their task with their software of choice.
As for Gimp's UI, i like it. It is consistent with the GTK toolkit, easy and clear. Granted it is not perfect, but the interface is not at all a problem unless you are a complete Photoshop addict. And as a side note, I've gotten hired explicitly BECAUSE I had Gimp knowledge. So it is not wasted effort learning it.
