Question for anyone who actually plays... Halberdiers or Spearmen? What size units of each work best?
Assuming 7/8th ed rules.
7th to 8th are very different. honestly, dont play 7th. well, no, hmm... okay. 8th is a new era of fantasy, 6th and 7th is an era, 4th and 5th is an era. 6th is better out the box than 7th, so unless you are putting a fair bit of work into some house rules, play 6th. if you are house ruling, you really want army books from 6th as a general rule, and the system/magic from 7th and it works pretty well with a few fiddly bits.
8th is a different beast, much more magic and large blocks focused if you want to win tournaments/be competatitve. If you are playing for fun, agree no 6th spells (the game deciding ones), and limit hordes (either no hordes or max one or something agreeble) and honestly 8th is in my opinion the best rule set fantasy ever got to (having played from 5th - 8th when they were out and 4th as a retro event).
So onto the actual question, in 8th, halberds are better, but both have a role (spearmen actually compete with swordsmen). Halberds are cheap str 4 attacks that are there to grind units down. Therefore, 40 strong horde formation, throw a character and some buffs on them and they are criminally good at grinding down things. I would even consider 50 strong in larger games (2400+ pts). In smaller games (less than 2000) or as a secondary unit not your main battle line, 30 strong is pretty decent too.
Spears are a bunker unit/bus. they will deploy 5 wide and (ideally) 6,7,8,9, or 10 deep and use steadfast with a BSB nearby to tie up enemy units. due to 8th ed step up rules, the extra rank of str 3 hits isnt worth it, so you are mainly taking them to provide wizard bunker (20 strong 2nd line unit), or 30+ as a tarpit. They compete with swordsmen who take the same role but with a 5+ sv and 6+parry they tank better, plus the WS4 means a few enemies out there will only hit on 4s rather than 3s. Generally I would take swordsmen over spearmen.
In 6th/7th, spears in units of 30 are a good line regiment as the extra rank of attacks is good as there is no step up rule. Swordsmen are still better but spears arnt bad, and due to the lack of armour, low Int and no extra ranks, halberdiers are far behind the other two.
So in short, swordsmen are offensive and best in 6th/7th, and defensive best in 8th, spears are defensive best in 6th/7th and not good in 8th, Halberds are bad in 6th/7th and best offensive in 8th.
Hope that rambling helps!