Good sleep and exercise will do that for you (speaking from experience).

For me personally - a significant difference. My productivity has increased due to rarely incurring cognitive fatigue due to fluctuating energy levels (i.e that afternoon slump after lunch goes away, skipping a meal has little or no consequence). I can code for 8 straight hours without having to eat where before if I skipped a meal I'd start getting hazy.

Additional benefits include weight loss while eating as much as you want, never getting hungry, being able to do a 10km run on an empty stomach and still be full of energy ..

Depends. I find myself more alert for some stints of time while in a ketogenic state, but on the other hand, have difficulty falling asleep (as in, normal day I'm drop dead asleep at 11-12pm, while in keto, I'm sleepy (very sleepy, groggy) but it is common for me to be awake at 2-3am. Wake up times is the same.

Not sure what is best to be honest since lack of sleep brings a lot of problems by itself.