Different guns fit differently.
Since they are for the most part the same underneather I would recommend you choose based on ergonomics first. I personally find that ARs are the most effective gun for me to shoot. Because the of position of the mag-catch I can reload faster and with less mistakes under pressure (this becomes really important for CQB). H&K G36s are similar, but MP5s are very slow for me.
Also there are a few schools of thought about how an AR is supposed to fit. I subcribe to what is know as "nose to the charging handle" meaning you keep the stock adjusted as short as possible without having your face riding on the receiver.
Whatever you choose, handle many before you make a decision.