First, there's about a 45 day difference between their release dates. It should be possible to get both eventually.
With that said, I would go for Splinter Cell: Conviction if only because it will be a much more polished product. Alpha Protocol is being produced by a developer that, while they produce excellent games, simply do not have the technical expertise to polish their products and remove bugs and glitches. Ubisoft is more proven in this regard, and they will create a game that will function more smoothly.
As for actual content, both studios are trying to do something radical, so we can't completely judge content yet. Conviction has a demo out and frankly, the game just looks really fun. I understand there's a lot of hate from the SC community about the direction the franchise is headed in, but I retort with the median-voter theorem. Ubisoft is pulling the franchise towards the middle ground between stealth and action, grabbing more players in the process; just as a politician in an election moves towards the middle ground between liberal and conservative to attract more voters. Ubisoft has been making this shift ever since Pandora Tomorrow came out, but it was far more subtle. Some might call Chaos Theory the pinnacle of the series, but I call bullshit. I think the previous games were limiting in what Fisher could and couldn't do. I might be one of the few people who liked the previous games to like Conviction even more, but I'll defend that point to the end.
As for AP, we simply don't know too much about the game. It's an experiment that can either boom or bust, but at this point, it looks fairly average.