They are both very close, yes you could use either one without a problem in this example.
The difference between the two?
I would say "hard" in most cases refers to objects better than people.
"The test was hard." Puts thes focus on the test as being the issue.
"The test was difficult." Puts more focus on the persons personal view or impression of the test being hard for them.
An example where they are not very interchangable:
"The dog was being difficult." Works
"The dog was being hard." Wrong
Chris
http://www.hearseesay.com