longer = faster but more nose heavy. A 14 to 16 inch will do well; as we have 16's in the ar's and they really aren't even 100 fps under my 20 inch. My 2-250 Stryker is a 14 inch, and it looses 300 fps or more, but it has more case capacity too. For what a 223 can do, I'd say 14 inch.
More length can't hurt anything. It has been a while since I ordered a barrel from MGM but they used to have one price for a barrel I think up to 15" then anything over that length got more expensive. If it were me I would go with the longest I could get for the lower price. Pay attention to barrel twist. If you what to shoot way out there go with a slower twist like an 9 or 8" so you can shoot the heaver bullets but still do well with light as 50 gr. for more explosive results at closer ranges.
well i opted for a 12'' after shooting a friends 14'' i just didn't want to haul at that weight around and glad i did , as the MGM pro-hunter tapper is a heavy barrel . also my barrel loved 22 grains of H322 and 50 grain Vmax . Very happy with how it shoots and feels .