first off....spears work the best in 20.
also .... and even more importantly, swordmasters also work best in units of 20 rahter then support units. Please note that this it not theory but rather experience. (also note that my tactica i mention that SMs work better in support units, but i would like to change this, as my views on them in the past 6 months have changed)
SMs are still infantry, and only (if ranked 6 wide) have 7 attacks. Compare SMs with a 6 strong knight unit. Beside the higher Ws, SMs really have the same effectiveness. Now. we all know that it is not smart to charge small knight units into the front enemy units. so why should it be okay to do it with a small unit of SMs (expecting to win from kills rather then static combat res)? The answer is this: Its not. The reason silverhelms and DP are best taken in small units is for rank negation, counter charging **this can only be done because of their movement**. Swordmasters on the other hand, are not as mobile, and are therefore hardpressed to get a flank. Additionally, when excepting charges, it guarantees that even elite units will be hitting on 4s....pretty good when you are trying to limit the amount of casualties the charging unit is getting. Even better, your St5 might actually be able to cause a wound in return....and in subsequent turns even better, as you will be striking first (also the turn your support unit charges in the units flank and negates his/her ranks). Therefore, it is much better to spend two more points per model then a spearmen to get a WS 6 ST5 guy accepting a charge rather then WS4 S3. Go with the rank and file swordmasters....take a unit of 20. They are infantry, get rank bonus + outnumbering combat res, thats what infantry do best!!!:yes:
Thus said, it is important to remember that swordmasters are still T3 AS5+. This means you wont be standing up to chaos knights/ warriors, or anything that hits really hard. If a chaos knight unit is preparing to charge any infantry unit, eagles can be put in front of them on an angle in order to redirect them as you wish. From there you can get either a flank attack or an additional round of shooting.