Mounted sarges are cheaper and can still do a number on infantry with repeated charging.
Imperial knights don't look that great statwise but they have armour piercing maces which make them better against armoured targets than feudal knights.
Strategically, I find that securing Northen Italy and Denmark early on to be very useful. It wipes out some of your smaller neighbors and enables you to focus on other fronts. This avoids the 6 or 7 way crunch that often happens to the HRE come midgame.
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