I will be using a damper actuator which can have either an AC or DC power supply. I do not have any strong preference for either, so what would you recommend? I assume that transforming 230V AC to 24V AC will be more efficient than converting 230V AC to 24V DC and thus will waste less heat, right? Are there any reliability differences?
It's not necessarily true that using a transformer to make 24V AC from you 230V AC power line is more efficient than a 24V DC supply. Modern DC supplies like that are switchers. They also have a transformer inside, but operate it a much higher frequency than the 50Hz power frequency the bare transformer runs at. In many cases the whole switching power supply can be smaller and lighter than the big 50Hz power transformer.
The big transformer will likely have higher quiescient current than the switching power supply. Under low loads, it's quite likely that the switcher will be more efficient. At high loads it's probably more of a tossup. The reason is that since the transformer in the switcher is running at a few thousand times higher frequency, it is inherently smaller and a little extra size and expense can be spared to make it more efficient. The same tricks could be applied to your large 50Hz transformer, but would make it so large and expensive that few people would buy it.
So as usual, check the specs. The big-iron transformer may win on simplicity and maybe reliability, but the switching DC power supply probably wins on efficiency and size.
Yes stepping down from 230VAC to 24VAC would only require a transformer and would be pretty simple and efficient. You may need to do some current regulation as well.
If all you need to do is turn on a solenoid that is probably the best way to go. If you are building a circuit and you need logic control that runs off of 24VDC (or some other DC voltage) that it may be easier for you to just use that.