I like to mix it up depending on the density of my opponents, if they are spread out I just go from couch kill to couch kill. If they are mixed up I go for the thrust, because you miss too many kill opportunities otherwise. Some times I ride tightening circles, couching off the straggelers to create a "bait ball" then I just ride through it thrusting left a right, great fun.
Couching is also excelent for taking out enemy horses if you just want to unseat a bunch of knights or Khergit lancers. The lance/polearm doesn't come out of the couch position if you're only killing horses, so I just ride accross the heads or rumps of groups of horses and put the riders on the ground. I think my best is killing 5 Caravan Guards horses off a single couch, before I accidentally hit an actual caravan guard and my couch ended.