My favorite of multiple macro lenses is the Sigma 180mm f/3.5 HSM-EX APO Macro. I bought mine used for $500. It is long enough to use easily for insects and butterflies,and it gives enough distance between the camera and a flash unit to make flash exposure control relatively easy,accurate,and repeatable. One of the biggest drawbacks of using a shorter macro lens, like a 60 or 90 or 105mm macro lens is that, as the lens is focused closer, most lenses will lose effective focal length, so the image gets smaller, and the working distance from the front of the lens to the subject is as short as 4 inches. At such short distances, due to the way light behaves according to a thing called the Inverse Square Law, changing the flash-to-subject distance as little as even 1/2 inch at 4 to 6 inches means that a flash that is mounted on the camera or the lens (like a ring flash) will have a HUGE fluctuation in flash power,and it sometimes becomes a major PITA to fully utilize the flash with the shorter macro lenses. This becomes an especially big problem when working outdoors in bright sunshine during the spring and summer months, when you start running into issues with bright sun, top flash synch speeds of 1/200 second, and camera ISO settings of 200 or so,and the range of f/stops you want to use.
You ask the question if it is better to have a 180mm macro or a 70mm macro. Flat-out, for insects, butterflies, snakes, lizzards, frogs, flowers, etc--the longer macro lenses in the 180mm range are "better" in terms of the working range and most people's idea of how they want to work. It's a LOT easier to get a high-magnification close-up shot at 18 inches than it is from 4 inches. it is easier to light with the camera,and your body not right on top of the subject, blocking the light from the sun or sky.
In terms of actual products on the market, Sigma's newer 150mm EX Macro lens really delivers beautiful images; I have seen hundreds of photos shot with the Sigma 150, by people from all over the world, and the image rendering (the bokeh and drawing style) of the Sigma 150 are really,really amazing; I wish I owned their 150mm macro.