Go for the S.I.(System Integrator) Companies if you want to build a strong foundation. Lots of things you can learn and develop your skills. S.I. do implementations that starts from the scratch and as well as troubleshooting... You can be a SWAT or navy seal (hehehe)
ISP companies needs a strong foundation and indepth knowledge of a whole infrastructure...
anyway, bro its up to you if where do you decide to go...