The difference between the trumpet and the cornet is that the trumpet is used to create a brighter tone. Cornets are used for a more dark tone, but actually they are both the same size. The cornet is actually more related to the french horn so in some others opinion it could be harder.
The B-flat Pocket trumpet is in the same octave and tone as the B-flat trumpet only smaller. It takes no less or more air to play, and if you have a good pkt trumpet it will sound identical to the B-flat trumpet.
The piccolo trumpet is tuned one octave higher than the trumpet. They are more rare than a pocket trumpet but I have gotten to play one and they are a little harder to play so I reccomend starting out with a B-flat trumpet before switching to the piccolo trumpet.
As for the baritones, it can actually be harder to play because you use more of your lips.

i play both and the cornetis smaller andthe sound is different some say it is smoother but the shortness is all i see.

Good Reply missu. I agree. Also playing a Baritona gainst a small brass instrument are harder and easier in different ways. I have difficulty adapting to the bigger mouth piece and tend to "lose my lips" but thats just because I was trained on a Trumpet and cornet and told not to blow your cheeks out. anyway Brass is best