Don't forget to calculate the total price for your budget flight, not just the fare. Most budget airlines in Europe fly out of secondary airports well outside major city centres, so factor in transportation to and from the airports, too, as well as the extra time this will take. Also, European budget airline tickets are bare-bones, so don't forget to factor in additional fees for baggage, seats, even paying with a credit card.
YMMV but I've found that sometimes trains -- while seemingly more expensive at first glance -- can be better value.