Alpine. I don't own either but both my HUs are Alpine and I wouldn't want anything else.
Alpine. Nothing wrong with Pioneer but Alpine are much better.
Better as in sound quality ? Their around the same price the alpine is more quality looking, but i think the pioneer is a new model and might be more future proof like if i change my phone i know they both work with my iphone 4s, not sure about the alpine and the iphone 5/5S
Alpine have a reputation for excellent sound quality and it is well deserved.
What I would say is if you can get a 2nd hand HU you'll get a good one for the money you're spending.
They're going to be about on the same level in terms of sound quality, price point will generally fix the quality of the components.
Down to personal preference really, might as well just pick the one that you think looks the best or has a feature you might want.
Or have a listen, they're likely to have their own unique sound signatures from different eq processing. As its not like you're going to be listening with it set to a flat response.