Depends what its intended purpose is. Is it going to be a bench gun? Are you going to hunt with it shooting from the Prone position? Will you hunt with it from the sitting position? You can get away with the shorter version on a bench gun. I would go longer to hunt with it from the prone position. If you like to sit and hunt, than you will need a much bigger bipod than that.
What size magazines do you typically use? I use a 9-13 Harris and it's just right for prone and on lower benches. Also, how much belly you have can affect how low you can get on a rifle too, I'm thin and don't need to extend the 9-13.