what are you trying to shoot? you did not mention anything so i am going to put whatever i can say on the subject.
those 2 macro lenses do 2 very different things
105mm is great for bugs because it gives you a lot of working distance so you do not scare the bugs away. it also gives you more flexibility with lighting. this is also an FX compatible lens so that if you ever decide to shoot film or go FX then this lens will still be relevant. the con is that 105mm is a bit on the long side so your pics can have a flat look.
40mm in my opinion is TOO SHORT for bug work. for full magnification of 1:1 the tip of the lens is SO CLOSE to the subject if you are shooting buys, it would have flown or dropped to the soil. lighting will also be a big challenge for that lens because you have to be so close. while it is a DX lens, many people actually use this for FX with acceptable results in the corners. this will also autofocus with your camera so it is great. now, if you are to ask me these 40-60mm macro lenses are great for food and product photography where i use them because the perspective is not as flat so that your products and food will not look small and your image will look more dynamic. now, you do not need to shoot real close when doing product and food pictures but there will be times when you will, like shoot the strawberry on a cake or in my case before, the beautiful engraving on a ring or a pearl pendant. this is also great for shooting watches (i came from a family of jewellers and watch repair/mod) because it makes the watches look "dignified" whereas a 105mm will make it look boring.
in dental photography, the best should be 60mm because it is closer to "normal" and with that focal length, you will be able to frame and see the other teeth. this is important for the laboratory so that they know which shade we are going to use for the porcelain and glaze to make the denture look like the rest of the natural teeth.
here are some of my bug pics taken with a 105mm so you can judge for yourself what a 105mm shot looks like.
https://www.flickr.com/photos/31768064@N03/sets/72157652297115512https://www.flickr.com/photos/31768064@N03/albums/72157652928071889there are few fields of photography as dependent on lighting as macro photography so lighting should actually be your 2nd consideration. i recommend a ring flash for convenience and this is the best one i saw, cost performance wise.
http://richardhaw.com/2016/01/24/study-diffraction-on-the-105mm-vr/and this is my writeup on the 105mm VR's diffraction effects. never mind the pictures, just read the commentaries.
you can still shoot macro photography without a true macro lens by using a reversed lens or a lens relay setup or bellows. these are very different approaches so it is out of this topic. for that i go to other dedicated macro sights like coinimaging.com and extrememacro.com (check the URL i am not sure about these)
