It depends on what you want to use them for. 6 wide with spears mean that you can potentially hit with 13 + 2,08 attacks from front row and 12 additional attacks, so a total of 27,08 attacks (the ,08 is useless though haha).
Comparably the sword and board guys will do 11 + 1,76 attacks from the front row and an additional 5 attacks from the second rank so a total of 17,76 attacks.
The issue is here that the spear guys are situational (spears doesn't work when they charge, only while being charged) and the sword and board guys have an additional save (parry save). This can worth it in the long run is why you usually have the S&B guys deeper so they can soak up attacks (more ranks = steadfast for longer).
Normally I go 30. That's either 6 wide and 5 deep or 5 wide and 6 deep. You can easily go 24 models, but at 2500pts I find them to die too quickly. Remember that they almost always hit last and doesn't have a good WS.
I actually think you can make a decent unit out of spear saurus wide a scar vet. He'll be able to chop up the tougher targets and the saurus will tear through anything else. But that's just me though..