Believing the one or the other is anti-scientific. The fact that we don't have any evidence leaves it unanswered though I'd argue it's more healthy to believe that there might be alien life because with that rational it would be easier to find it because you'd spend more effort doing so.
As for the subject of greys. They most likely exist. Denying categorically their existense is calling the total of myriads of people who have encountered them liars. Are they extra-terrestials? Maybe not, they may be intra-terrestials and with it goes the theory of hollow earth, its inhabitants and its litterature. Of course it carries the stigma of being solely a literature with minimal, if any, scientific backing which brings it vis-a-vis to a disadvantage to conventional science and its solid earth theory.
Then there are the theories of the conspiracyologists, like Icke's Reptilians and Von Daniken's alien "Gods" which for the most part I find dubious because they practise selective literature review combined with "enlightment". But they may have pieces of truth and it's not useless to keep them at the back of your mind. For instance, I find fascinating Icke's claim that the Reptilians have an addiction with symbols and rituals. That fits the 2012 topic and how it is a key year for the destruction (of the old) world. Also some very, very suspicious perseverance with the so called "holy geometry" that we keep confronting everywhere, Nazca lines, Statue of Liberty being in reality a depiction of an ancient Babylonian godess etc.
At the possibility for any alien civilization outside our solar system there is the strong arguement of vast distance and the barrier of the speed of light. Besides not a very great speed if you consider the magnitude of space. The counter arguement is that they savagely bend the fabric of spacetime and instead connect remote areas with wormholes using huge energy generators such as black holes for the process. Their play with time (or rather the rate or rhythm of energy flow as I argue in the astronomy thread) explains also the radical movements of UFOs that defy the laws of inertia and supposedly kill any living beings inside of them.
This is solved because they operate inside a shield of a different flow of time. Instead, they look on us in slow motion yawning as everything procceeds in comedic slow motion and they turn at safe speeds. This would sound absurd a century ago but no more after Einstein published the general relativity. For more on the subject on antigravity and spacetime manipulation watch these series
http://www.youtube.com/watch?v=nP4zQ4R8vJg Though he doesn't really produce antigravity but uses soundwaves on liquids to illustrate it. Anyhow very interesting.
I have a picture of me little in the woods and on the sky there is a strange round aircraft. Pity the quality is not very good since it's not digital, I believe it was Polaroid. I'll upload it when I find it.
The truth is out there.