Originally Posted by Maelochs

There is more than one right answer, is the answer. Your bike likely has a bigger biggest cog than theirs so you need the longer cage.

The question you might want to ask, is "Do I use the top one or two cogs on my cassette?" But I wouldn't worry much. For about $100 you could get a 11-28 or 11-25 and a short cage derailleur and do actual scientific tests, riding the same route day after day, using the same rest, recovery, and nutrition and seeing if there was a difference. I would bet any variation would be completely within the realm of experimental error.

Your bike is different than theirs in that it has a wider-range cassette. That is the real difference. The long-cage derailleur is necessitated by the wider cassette. Next move is yours.

me, i'd just ride.