Merceneries for me, mainly because they are quick additions to my army and I dont care about them much. Especially when it's jsut a wooden wall city, usually I end up with the bottleneck gate. So I have 'waves'. Usually the mercs are heavy infantry, but I dont just sacrafice them, sometimes I'll back them up with light infantry such as Hastati. Then if there's alot of resistance and the enemy havn't fleed to the central square, then in go the principes.