wordpress website multisite or not?

Hi,

im looking to create a website.. which is just a portfolio of a business.. but they have different departments under their main business.. think of it as a software house.. they have ios dept, android dept and enterprise solutions.. now they want to have different design for all three different departments.. and may be have a sub domain allocated to each dept. BUT their main domain will remain the same.. (different sub domains for each dept).

i dont know if i should go with multisite approach? will it allow separate theme for each dept? will it help with seo in future? how to have a navigation on sub domains (automatically) to go back to main or other dept.

your valued input would really be appreciated.

thanks.