Which is actually better on system resources?
That all depends on just how far you want to go in terms of detail. If you intend to have all the hull plates and other details look 3d, go with a normal map in preference to modelling in that detail, because if you modelled it all in you'd be looking at thousands and thousands of extra polys.
However, normal mapping is currently best suited to
only that kind of detail. If you need larger or more complex detail, then you'd be much better off modelling it in. An example of this is the Arcadia - even its small detail is too big to accurately portray using normal mapping, so I'll be modelling it in using detail boxes and stuff.
Basically: modelling it in is far less resource intensive than adding a normal map, but only up to a point. Beyond that point you'd be wasting thousands of polys where a normal map could be used instead.