The soldier class feels more like fighting an elite while the Knights feels more like hunters. The Spartan abilities like thrusting, charging, ground pounding allows for different play styles against them making it way more exciting IMO.
One thing I do just want to point out is that there is a slight learning curve at the beginning. Especially with this being the first Halo game being made in 60 FPS from the ground up and all the awesome Spartan abilities...it took me like 10-15 minutes of fighting to get the complete hang of it (I've beat every single Halo game on Legendary as well...so I'm definitely not new to the series)...but after that I was in FPS heaven. Game runs fantastic.