I'm pretty sure your odds are the same.  Pool difficulty is less than the network difficulty, but every time you find a share you are solving the pool problem.  The pool just compares your result against the network problem to see if you solved the block.

But just because the odds are the same, doesn't mean you will find a block solo mining after 1 week.  There are calculators out there that can tell you based on the difficulty and your ghash how long it SHOULD take you to find a block solo mining, but that doesn't mean that is how it will turn out.

People tend to use pools because they equal steady income over time.  Solo mining is more like playing the lottery.  But in the long term (years) they should just about equal out.