I think ballistas are more useful, at least in the early game. They fight well alongside crossbowmen, but with spearmen, you'll suffer heavy losses. Their defense is barely better than that of mages, and the attack through the front row doesn't help much. Ballistas are ideal for sieging cities and for defense as well; if they're placed on a tower, you're in good shape. Besides, if you do decide to build spearmen, you'll have to build monks later, and they're less useful than ballistas. Crusaders are much better than spearmen. Medusas are more practical and useful than minotaurs in the early game, but I still prefer minotaurs. Like ballistas, they have an ability that's effective against any enemy. I respect golems and build them more often, although mages are one of the best units in the early game – while halflings with their accuracy decimate neutral level 1 creatures, mages raise skeletons, which finish off the rest. But in serious battles, mages are practically useless, so golems are better.