The steg is really designed to get off the charge. It works in synergy with small skink units that can bait and flee or act as chaff to set up a unit for a charge. For example, put a 60 point unit of twelve skink cohorts in two ranks in front of a monster or tougher unit and shoot with javelins for poison hits on 6, then stand and shoot with quick to fire against to tke the charge. You get 2 poison hits in shooting phase and another 2 in stand and shoot. If you get one or two unsaved wounds on the monster and hold on steadfast, then you hold the enemy unit up and hopefully can counter charge. More likely, if the skinks break and flee or are destroyed, then the monster or unit can only pursue or overrun or reform. If you position the steg about 11" out from the front of the skink unit, then the pursuit or overrunn will likely fall short or the steg (monsters do not have swiftstride) and the ability to countercharge the monster with the steg is huge. The same tactic works with demigryphs and heavy cav and similar units.
The key to the steg is getting off the charge and then being stubborn, having terror, taking few wounds, and holding up something until you can deal with it or flank it. The ancient steg's ability to hit hard and break the opposing unit is the key and then hold with stubborn if the impact hit roll is low and the hits are low. Remember that that unit gets five crew attacks and 3 steg attacks in CC. Also, the steg needs to be supported by a unit of skrox, TG or saurus that catch up.
Since a lot of models are T4 and some or T5 or T6, even T7, and/or have very high armour saves, the extra +1 S really makes a big difference in the damage caused by the impact hits and cc hits of the steg. One less cc attack on the ancient steg is worth +1S on that steg. Also, if the goal is to potentially hold up for a turn with stubborn until the support arrives, then the extra +1 AS is worth a lot. S3 wounds on the steg are unsaved 50% of the time on the steg and 66.7% of the time on the ancient steg. Thus, the ancient steg will survive a bit longer and hit a lot harder.
There is another aspect. I am a fan of using the steg to go after ranked infantry with S3 and S4 attacks where steg thunderstomps do a lot of damage. In that environment, I'd take the two giant blowpipes with 2D6 shots at S3 and poison with the new range of 18" over the single giant bow if I already have an EOTG steg. That is 4D6 poison shots at 18" range. You either have to avoid moving or shoot at close range of 9" in order to hit on 6's due to the multishot special rule penalty. So, the shots ae best when the steg is out of position to charge but already close in to battle and being used to set up a flank or rear charge (assuming the target unit does not turn around).
Also, if you are taking sharpened horns upgrade based on expecting to see monstrous units and monsters, lone characters, and chariots, then you really want an ancient steg for the S6 because your charge targets are likely T4 to T6 such that S6 matters and increases the number of wounds both due to grater wound probability and more likely to cut through 1+ to 4+ AS models.