Torn between two trips....

A friend and I are planning a trip to Europe the first two weeks of May. We are trying to decide between a trip to Ireland/Scotland or Italy/ Croatia. We are more interested in seeing the countryside than the big cities, and want to do some hiking, biking and other outdoor activities. We want to travel relatively inexpensively, and plan to stay in hostels and limit eating out. We may rent a car for at least part of the trip. So, I'm really looking for advice from people who have visited/ lived in any of these places- cost,transportation, things to do- I could use some help deciding which trip to take!
Thanks