in my experience:
electronic filters, internal or external, are a whole heap better than UG filters. an electronic one will collect the 'gunk' out or the tank into itself, which you then clean out.
UG collects the gunk under the plastic platform. you then have to try and get it all out without disturbing the tank.
when i was using an UG filter, i had major problems with nitrate levels being very high no matter how many water changes i did. it cleared up as soon as i took it out.
you can use both together.
as for carbon, i find its a waste of time. yes, it does absorb chemicals in the tank, but if not changed reasonably frequently, it can release these back into the tank. for an UG filter i used to use zeolite.
BUT, as i said, this is just my opinions and experience. everyone differs