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I know they both furfill the same role but does anyone even use more than 1 block of an anvil unit?
The thing is I definately want to use PG but I already assembled and beautifully painted my spearmen.
Viable builds for them?
Actually, there is no reason to only have a single anvil regiment. The PGs are going to be the best anvil, and the guys that you would want to put up against your enemy's hard hitters.
Spearmen also make decent anvils, but without the Wardsave, or the ability to cause Fear, they're not quite as sturdy. You need some minimal core choices though, so if you don't take archers, why not take spears?? Against Dark Elves or other low-T enemies, they can be devastating, since they have ASF and fight in 3 ranks.
Seaguard are a bit of a tough one. They're pricey, and if you're just looking for an anvil, you should definitely take either these or PGs. LSG are the best for an anvil in a low point game, which actually fits their fluff of being intended for deck-sized battles aboard ships. Against low-T enemies though, you might be tempted to use them in larger games, where they can do tons of damage with their Stand-and-Shoot before they attack with their ranks of spears.
Spears and LSG can both work as minor anvils to set up flank attacks from cavalry, elite infantry, or chariots. They can also act as a stop gap, filling up space in your front line for your archers and warmachines to hide behind. They do certainly fill a role in the army, so don't worry about not being able to field your spears.