Originally Posted by abduraooft
Values saved in session for one user won't be available for another one
Of course it won't.
But I was thinking of the situation where a user might go to your page containing the form whose select lists are populated from the db. If the user then goes to another page on your web site and then returns to the page containing the form, the page will have to make another call to the db to retrieve the 300+ options instead of getting them from the session array of options.
Regarding using an xml file, I'm pretty sure it would be faster than using db but an xml file, being text file, will most probably be less secure than a db.