As Mike has mentioned working distance is an important consideration especially if you are intending to shoot flighty subjects.
Working distance will increase with focal length. Also as focal length increases field of view decreases which can be useful for limiting the impact of distracting backgrounds. Example the field of view, or picture angle on the wide side, of the 55mm Micro is 43degrees, the 105mm is 23degrees and the 200mm is 12 degrees.
http://www.jeffree.co.uk/pages/macro-lens-calcs.html
The chart at the bottom of this link gives values for a number of lenses. The chart is split into two sections. The upper section is for 1:1 magnification ( life size at full frame ) and the lower section for 1:2 ( half life size full frame ) Working distances are in the 4th column
WD at " R ".
I have the 55mm Ais Micro Nikkor, and if I had my choice of purchase over again then from my experience of changing subject matter I would go for the longest focal length I could afford.
As you have seen quite a number of members favouring Sigma.
If you can handle working in manual focus then it is worth considering a used Ais 105mm or 200mm if you can get one.
Don