the best price is one you can afford.
120GB should be quite a good price ... and could be either 2.5 or 3.5"
2.5" drives are more portable ... generally don't need a powerunit .... and (this is a huge generalisation!) will be less long lived (because they won't have extra cooling ... and are more likely to be knocked about).
3.5" drives are big and heavy, need external power - but will tend to live longer.
as for brand ... I'm sure you'll get lots of suggestions ... stick with the big names for reliability ... the less well known for a cheap fix