My preference has always been for a true, fixed focal length (prime) macro lens - the longer the better. A longer focal length gives you greater working distance between the end of the lens and your subject. Having that extra working distance makes lighting far easier for inanimate objects such as you wish to do. It is also a more comfortable working distance for animate objects like insects, frogs, etc. that won't tolerate a shorter working distance.
Given your decision, I would recommend the 105mm. However...the 150mm Sigma would be even nicer to work with and is about $100 more than the 105mm. Again, greater working distance is the key, especially with lighting, both artificial and natural as in both cases there is more room for reflectors as well as main light source.
Using extension tubes on your current zoom will work - and will work fairly well especially zoomed to 120mm to get that greater working distance - but will not have the sharpness and contrast of using a true macro lens which is specifically designed for closer focus. If you have the money and want to "go macro" then saving for the 150mm would be option 1, I would think; the 105mm would be option 2. Without testing optics, you might find that your current zoom with extension tubes will perform almost as optically well as the 70mm but at a slightly greater working distance. Of course, it's also important to consider that given the nature of your photography and where you wish to go with photography it might make it difficult to justify the additional cost of a macro lens over extension tubes.
Photography is a trade-off: higher quality mean greater cost. I alway advocate saving for the best you can afford. But, if you are paying for quality that you will never really use, then maybe it's money you don't need to spend.