There really is no "best"..... it all depends on the particular situation
and usage at the time. Generally mobile units have more power, better range
and longer lasting power sources (car battery).... but you are tied to a
vehicle or must lug a heavy high amp hour battery around...so they are not
nearly as mobile.

A portable has limited power but this often isn't a factor unless you are
far from a repeater. The beauty of a portable is it's ability to pretty
much go anywhere.

In short you can do things with a mobile you can't acccomplish as surely
with a portable.... but under certain circumstances the reverse is also
true. I'd have both.

Mike