The rear derailleur is an "arm". A large diameter sprocket requires a long reach to get round it. E.g. if you want one with more than 30 teeth, you need a longer arm.
You tend to get big sprockets on mountain bikes where you need nice low gears. A road or TT bike will have much smaller ones - and quite a small range between the smallest and largest, so a shorter arm is needed.
So, you probably need a shorter one, unless you have realllllllyyyyyyy looooooowww gears
You need a long arm if your chain has to cope with wildly different cog sizes, like Jack says. But he was only talking about the back... you'll probably need a long arm if you're running a triple at the front, too.
The arm has to take up all the slack when you wimp out and go for the granny cog.
Comments
The rear derailleur is an "arm". A large diameter sprocket requires a long reach to get round it. E.g. if you want one with more than 30 teeth, you need a longer arm.
You tend to get big sprockets on mountain bikes where you need nice low gears. A road or TT bike will have much smaller ones - and quite a small range between the smallest and largest, so a shorter arm is needed.
So, you probably need a shorter one, unless you have realllllllyyyyyyy looooooowww gears
The arm has to take up all the slack when you wimp out and go for the granny cog.