mysql or oracle 10g Express

we are creating a web application and going to host
that onto a customized operating system which has mysql embedded in it
we are yet to decide whether to go for that or oracle 10g Express Edition which is also free .
if anybody can tell me just the differences between them
i will be thankful

you should know that Oracle Express has some limitation , for example in total amount of memory that it uses and total number of processor that it can leverage.

If you have no problem about the above limitation (and perhaps other limitation that are mentioned in its data sheet) then i suggest you use Oracle instead of even Mysql 5 :-) .
you will have a broad range of options when you use oracle that you wont have if you use Mysql.