Is MPLAB more professional (serious) or is it just bigger and overly complicated
I think it is no more professional or less professional than MCS. It can do things that MCS cannot. So it is just a matter of different tools for different jobs.

One of the things I like about MPLAB is it can tell me how the fuses are set after a program is compiled. I find this very useful when working with an unfamiliar chip when I am not sure it I have things correct. It also can take a saved hex and tell you how the fuses are set, nice when you need it.

Again it is a matter of the right tool for the right job, and what you become comfortable with. Some times I us XP or Linux, MPLAB or MCS, PICKIT1 or PICKIT2 or PICSTART PLUS. Seems the only thing constant is PBP.