This is a great question.
Gym based ME has the advantage of allowing the athlete to perform anywhere and has be developed with the endurance athlete in mind. The Outdoor ME requires that you have a very steep hill as it is not meant to envoke a cardiac response but is limited by strength so the athelete needs to keep that in mind.
I personally am like you and enjoy doing things outdoors and have had good success with rotating the ME from indoors to outdoors on a weekly basis and moving through the progession for each.
I hope that helps!