Hi Pete

from what i can understand, digital servos are alot faster and more accurate that anolouge servos but they suck much more battery. If the LT-40 is a trainer i would use the normal servos in it and put the digitals in the twist. Digital servos are really only needed for 3D and patten aerobatice where you need fast precise movements. for a sports model i wouldnt bother with them mainly because of the price.
I hope i helped

Dean