Hand held is the general order of the day.
I'm gonna get the 150 again.
mind you I have been doing quite well with my 70-200 f/4 L + 1.4tc and
31mm extension tube.:t:
This is something have never understood (buying a high quality macro lens and then compromising IQ by not using it on a tripod).
Please don't take my comments the wrong way, but I just don't see how you can get acceptably sharp results by hand holding a long macro lens like The two Sigmas in question, or Canon's 180. In my experience using the 180, or any long lens such as the Canon 100-400L, focussing for close-up subjects like dragonflies is
absolutely critical. Typically I have to use magnified live view (5x) to ensure I get the bits I want nice and sharp, as in the attached example. I was careful to focus on the area just behind the subject's head, but even at F14 & 1/80 sec, and with the subject almost parallel to the lens, its abdomen is still blurred.
Ok, so you will be lucky and get perhaps 1 in 10 shots where you just happen to hold the camera in the correct plane/distance from the subject so that the desired bits are in sharp focus, but mostly you are going to sway back or forward just far enough (less than 1cm is all it takes) to lose focus. Even a lens with built in IS cannot compensate for the DOF problems that are typical with DSLR cameras and longer lenses, as they only correct for lateral movement.
I have done a lot of this type of photography over the last 2-3 years, with 3 different long lenses and various combinations of tc's and extension tubes, and usually in typical UK light conditions.
Always I am fighting against a compromise between wafer-thin DOF and shutter speeds that are just fast enough to freeze any motion blur from the wind or movement of the subject (it's surprising just how much vibration is caused by wind, even on a 'still' day).
Interestingly I met a guy in Norfolk the other week who was using one of the older Sony bridge cameras (think it was a H1 or something similar. Recent models typically have about 10-12MP, 12-20x zooms and built in IS) to photograph dragonflies, and he found that it was far easier to get sharp pictures than with his expensive Nikon DSLR with macro lens without a tripod, because of the built in IS and the greater DOF (which is presumably a result of the smaller sensor?) So much so that he rarely carries the full DSLR kit into the field any more.
This is just food for thought, but surely having an expensive DSLR macro set up and not using a tripod is a bit like owning a Ferrari and only using it to commute to work through the city rush hour! In other words, you will never realise the full performance that it is capable of.
At about 1/4 the price of the Sigma 150/180 and a decent (used) body like a 40D, surely such cameras make a lot of sense if you insist on not carrying a tripod, not mentioning the lighter weight and ease of use. I think many of the later models also shoot RAW and HD video too! and make decent general purpose 'happy snappy' cameras for family occasions/holidays etc.
Steve