The best plan might be to try macro first with what you have (extension tubes, close-up lenses), and when you see what you will need in terms of focal length, get anything marked macro in that size. Your Nikon 105mm might be fine with tubes.
Shorter lengths may be better for some things, like small insects or stamps photographed on a stand, while longer ones may be better for butterflies, flowers, etc. Most people like to be able to get back (longer lens), but the shorter lens may be more economical. I used a 75mm Nikon enlarging lens on a Canon 35mm film camera (with bellows) for macro for years, and this was economical compared to most macro lenses. Of course, nowadays people want auto focus too, so that is out.
To be honest, most of my macro today would be selling something on eBay, where the finest photography is not required (but it helps in some cases). I have one prime macro lens that is also a portrait lens (Canon), 60mm for my Canon digital 650D (APS-C cropped sensor). If I want something like fine art in macro, I would consider a bigger camera. My signature picture here (the Rex Begonia) used an 8x10 camera with a big process lens (made for close-ups, like enlarger lenses--also rather economical even for the best), but the Canon attached on the back instead of a film back. That permits auto exposure, but manual focus.
The Kodak workshop manual, "Photomacrography," is still available online, and it makes a good textbook for pros and serious amateurs, even though the equipment technology is retro. The cover of that book used a tiny Kodak 16mm camera lens, the 25mm cine Ektar, mounted backwards on an 8x10 camera--using 8x10 film. These cine lenses are still sought after for macro--and micro (over 3 times lifesize on the sensor). I have tried that on the Canon (adapters made by S. K. Grimes) with better results than my Canon macro--and the Canon is not a cheap kit lens. But with that sort of setup, you must stage it with a tripod--it is not action photography like catching bees in the wild. In still life macro it is possible to test the shot with digital, then use larger film with exact settings.
So see what you need by exploring what you shoot, and how. If you really get into macro, you will probably want (later) to expand your options. For many people, extension tubes and/or close-up auxiliary lenses do what they need.
kcj wrote:
Want a macro lens for full frame Nikon I have a Nikon 105 but it is not macro trying to decide between Tamron 90 mm macro and sigma 105 macro which is better or do you think there is a better macro out there