The generic activity of sitting on a horse and moving can be expressed in most cases with both "sein" and "haben" (with some preference on "sein") as auxilary.
Ich habe im Urlaub ein Pferd geritten
Ich bin im Urlaub ein Pferd geritten
are both expressing the same thing. Both your example sentences thus mean the same thing to me, as they express this generic activity. I would, however, choose "sein" over "haben" in these examples - It's just more common.
The only case that I can come up with were "haben" does not work at all is reiten in the sense of moving yourself from one place to another like in splattne's example, when no object is present:
Letztes Jahr bin ich von Rügen nach Usedom geritten
There are a number of meanings of reiten, though, that specifically ask for "haben":
Owning, occupying or simply having a horse or other animal:
Beduinen haben früher Kamele geritten, heute nehmen sie Geländewagen
Ich habe früher einen schönen Schimmel geritten (meaning "I kind of owned it")
Moving the animal to a certain place by riding
Ich habe das Pferd auf die Weide geritten
- Causing a specific condition of the horse by riding
Ich habe das Pferd müde geritten
- Causing a specific condition on yourself by riding
Ich habe mir den Hintern wund geritten
- (Passive) in a figurative sense as "you must have been occupied by something"
Was hat dich denn geritten, als du dir dieses Auto gekauft hast?