Interesting question, to which there's only one answer in my mind... it depends.
A concrete pier can be a liability when you go to move. Oddly enough not all people are astronomers so a beautiful sturdy concrete pier in the middle of the yard may not be a strong selling point... but a steel one you can at least unbolt and take with you and leave the concrete footing largely hidden underground
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The stiffness of a pier is proportional to the 4th power of the diameter. I just ran some numbers to compare steel and concrete
... For the same diameter pier a steel pier will be 10 times stiffer than the same diameter concrete one!
(assuming 20 MPa concrete and neglecting the effect of reo). That's a much bigger difference than I expected. Since the Elastic Modulus of concrete changes with strength, making the pier out of 50MPa concrete reduces the difference to a bit over 6 times stiffer for steel.
So one might think the obvious answer to the original question is that steel is better. Well, only if you do it right!
I've seen a lot of bad practices in the construction of steel piers.
Some tips for constructing steel piers:
- Go for the biggest diameter hollow section you can for the pier column. (not so big it fouls your OTA though).
- Don't use a section that is not a complete hollow section e.g. no channels, I beams, angles, etc. Only a continuous hollow section will give the torsional stiffness required. (there's negligible advantage in going completely solid).
- Minimise the width of both the bottom and top flanges! Too many piers I've seen have wide flanges from thin plate which become very flexible. Keep the flange plates reasonably thick and the width of the flanges to a minimum.
- Minimise the height of the pier. The stiffness is inversely proportional to the cube of the height.
- Grout the bottom of the pier to the footing. A lot of piers are installed without grout. This makes the bolts effectively a hinge at the bottom of your pier.
- Keep adjusting bolts at the top of the pier short and thick. Again they are another flexible point.
Unless you do all these things with a steel pier, I would say a concrete one is better IMHO.
Al.