I own both the Metz 58 and 48 flash units. There are
many more useful differences that the
two big differences outlined above i.e. the Metz 58 can do so much more than the Metz 48 can.
You can visit the
Metz web site if you are curious about the details.
http://www.metz.de/en/photo-electronics/mecablitz-models.html
If you are quite serious about flash photography
and a photography gear addict to boot... then the Metz 48 won't satisfy you in the long run... else it is absolutely perfect for you.
Keep in mind that most of the features in the Metz 48 are also found in several other brand flash units too... but some of the features in the Metz 58 are simply unavailable in the other brand flash units. Also, the Metz 58 is a really big-sized ultra-powerful extremely feature-rich flash... whereas the Metz 48 has a comfortable size and often (but not always) will allow you to get the job done quite satisfactorily when compared to the other brands.