I'd say....
#1-4K; It gives you more of a rush when playing faster maps and is often less complicated. A downside of it is for the less keys, there are more notes for each.
#2-5k; OK, OK, HEAR ME OUT. It gives the same rush of energy and accomplishment as 4k, without too many notes on each key and still not too complicated.
#3-7k;obviously... this mode has more of an extendability for options of gameplay and a feeling of self-accomplishment without the feeling of "WTF IS GOING ON"
#LAST-10K; I do not like the idea, gameplay, nor setup of 10K. There are way too many keys to keep track of, you have to use the weakest fingers you have (whcih ever one that is) no matter what, it gives your fingers a strained pain without a perfect posture and hand position, and I think 10K must have been made out of desperation as it's poorly thought out and practically unused anyways.