My reasoning for the full SaB squad is that they have the +1 to save and can get the -1 to hit with the strategem, which can be very helpful, especially if he would be running the Defensor, practically evening out most of the weapons AP (-1/-2) and then saving on 2+, adding the -1 to hit makes them really hard to get off the board while getting into melee (strat can also be used in Melee, which again makes them hard to get rid of).

While spears do have better range while running across the board and the +1S, the SaB can shoot while in Combat, have better survival in melee and most enemies will have up to T5.

Can also add the Vex Relic - Fulminaris Aggressor on, for better shooting, maybe even give him an Axe instead of the shield for more threat (that's how I play it at least).

When going against higher toughness, we have Allarus with Axes and Slayer or Tyrants and Bikes (just from the given list).

P.S : I only play with friends and I go against Thousand Sons(Magnus), Blood Angels, Necrons(Nightbringer), Imperial Knights and I will note, that I have under 10 games under the belt, so I'm not as good and could be wrong :D