you will just about always gain some velocity with longer barrels no matter what round is used , the longer barrel will allow you to use slower powders more effectiently and that will help get higher speeds with heavier bullets. You will get higher speeds with a 27" over a 26" and a bit more with a 27 than a 24" maybe a 100fps depending on the load .
So if you looking for the most speed from your rifle then get the longest barrel you can , but if the barel length is going to affect the manuverability of the gun then youhave to find a happy medium. With factory ammo I doubt that you would see a 75fps differance between the 27" and 24" barrel and if reloading you can adjust your load to make that lost speed up.
As for as accuracy goes , the shorter barrel will be stiffer that leads to less viberation and that generaly lends itself to better accuracy especialy over longer shot strings like 5 round groups over 3.
If it were me I would get the longest barrel that won't bother your moving the gun around , one thing that I have don't the last couple time I ordered barrels was have an extra inch added to the shank end of the gun , this does two things , one it helps move the balance point back toward the middle of the gun and two it will allow you to chop of more on of the chamber if you decide to rechamber the gun of the throat gets eaten up to bad.
Si Vis Pacem Parabellum