There really isn't a load out for the landa (mob wise) thats better than the other,it usually depends on the target your going after.
If your trying to take on a formation with small numbers and/or poor CC then the extra grots are not nescessary.Things like whirlwinds/artillery batteries/warhounds etc.
If your going to take on a formation with good CC or FF then the extra grots are essential.Stormtroopers/Marine infantry etc. I've even had Terminators taken out by a well timed ork air assault as the 4 grots took most of the damage the termies would have done and the advantage of numbers and BMs really took its toll.
If your regularly facing big formations like IG companies then try stormboys added to the mobs to give the option to jump out to 15 cm and cut the FF return down a little.
Most of the descisions of what to take will be easier when you've played a couple of dozen games with different load-outs and adapt things using a few points of reference.
a) what your opponent usually fields formation wise.
b) your own style of play as some players are more cavalier about the type of targets they go after (juicy formations that if destroyed will give a big advantage in the game but have a risk of defeat involved)and some are more reserved (going for softer targets like artillery to try to gain a tactical advantage for their advancing hordes).
c) how your opponent sets up their formations (in game not just at the start) has to be taken into account (a tight packed formation,base to base,will allow more attackers to jump into CC but a formation wide spread,using most of the 5cm coherency, will mean less of the attackers being able to get into CC but a better chance of a clipping assault).
d) as a few have already pointed out,what do you want the grounded mob to try to do afterwards.Are they just a throw away unit or do you want them to do other things once they are down.Nobs are better for shooting/assaults etc once the grots are gone. Stormboys are scouts so can be very annoying forcing indirect fire formations to have to move (10cm zone of control)and have a bigger move range for assaults or objective grabbing.
Also don't forget the other stuff that can go in the landa too,they all have a niche they fill,it doesn't always have to be a mob that goes in there.The warbikes are versatile for a landa,they are good in assaults with good movement for afterwards and have decent shooting if its too risky too assault.
Hope some of this helps you decide what to use and also gets you to try to keep your opponent on edge by not sticking with the same force every game and trying different or unusual combos
.
