The evidence is that evolution occurs and has occurred and that the earth is very old. As such, the only way YEC could be true is the Omphalos argument (which has the theological problem of God's dishonesty) -- that is, effectively God used magic.
Now, WRT aliens: By all that we know, the distances are too far from anywhere to travel where there might be life sustaining planets via sub-light speeds. Super-light speeds are impossible because one cannot accelerate to the speed-of-light and hitting the speed of light (as far as we know) causes a divide-by-zero error in the math. So either aliens know how to use worm-holes or they know how to build space-ships to sustain entire eco-systems and generations of beings while they travel. As it has been famously said, sufficiently advanced technology appears to be magic to a society not as advanced.
So, we have God -> magic and aliens -> magic. Magic/magic == 50/50.
At least the alien theory doesn't require that God lied to us.
Not to try and give credence to the argument... and not to hijack the thread... but do you really think it's THAT impossible that an extra-terrestrial species would have developed the ability to travel inter-solar system or intergalaxy?
I agree that faster-than-light travel is impossible... so don't think that I'm challenging basic physics here... but there's been plenty of speculation regarding how intergalaxy travel could be achieved. (TV references provided for those that haven't ever heard of these methods)
Star Trek ideas:
Warp-drive: Essentially, you have a ship that is encased in a field, and then compresses space ahead of it. It then travels across the compressed space. For visual aid:
Instead of traveling this far (pretend each hash is, say, 100,000 miles)-
|...|...|...|...|...|...|...|...|...|...|...|...|...|...|...|...|...|
You travel this far-
|.|.|.|.|.|.|.|.|.|.|.|
Worm holes: I'm not as familiar with the concept, and it's been thrown around a LOT in various movies and shows. Essentially though, you have an (optimally) stable "tunnel" through space that allows for passage from one part of the universe to another.
Event Horizon theory:
Folding space: Instead of compressing space, space is folded so that the origination point and the destination point occupy the same location in space. (Of course, if you believe the movie, there's a gap between the points where hell resides
)
Bottom line, each of these theories provide for "work arounds" of the faster-than-light limitation. Not saying I particulary believe any of these ideas would actually work... and hollywood certainly isn't the authority on quantum physics... but it's kinda fun to think about these possibilities anyway. (Or maybe I'm just a geek)