In terms of pure hassle-free first car ownership, the Fiesta will be a better proposition than the Saxo. It'll take more punishment, break less, and be cheaper and easier to fix when it does break.
Saxo will handle a little better but not really by any amount you'd notice with it being your first car.
As a final test - how many Mk3 Fiestas do you still see around versus Citroen AX's - the Saxo's forerunner?