SRAM rear derailleur arm length

[jacke]

I am riding a Bianchi 928 with SRAM Red components.  I have a 10 speed 11x28 cassette.  I am replacing my rear derailleur since the guide pulley cogs are now worn to sharp points.  The present derailleur has a short arm.  SRAM seems to have 3 sizes of derailleur arms for a 10 speed cassette; short, medium, and long.  The 28 cog gear still works will with my short derailleur.  I heard that you need a longer arm if you have a bigger gear but I am not sure if that is true or not.  Is there any reason I should not replace my short arm derailleur with a medium length arm?  Are there advantages or disadvantages to either?