I have developed games using both the android sdk/andengine , as well as unity, and my personal preference is to go with the android sdk route. Unity is great, but seeing as this was my first trip into the mobile development world, I took this as a good excuse to learn the environment, and understand the constraints and pitfalls associated with it. Unity simplifies everything so much, making everything from debugging to deployment ten times easier, but I must say that from a personal perspective I don't feel like I got a darn thing out of it.
If your goal is rapid prototyping, or you simply aren't interested in the challenges or opportunities to learn associated with the android platform, then you should go with unity hands down. Otherwise, I would recommend the SDK.