There are generally two types of domestic solar thermal panels.
Flat Plate and Evacuated Tube.
The major difference between them (apart from price) is the method of insulation.
Flat Plate ones rely on a small air gap between the glass and the collector element and some insulation behind the collector element to reduce heat loss.
Evacuated Tubes rely on a vacuum to reduce heat loss.

Flat Plate can give a larger area of collector element when compared to Evacuated Tube types, so the relationship between the amount of sunlight collected (irradiation) and the total roof space covered (m^2) is often similar.
This can therefore give a similar performance overall because the total amount of irradiation amount to the same over time (this is known as insolation).

Generally speaking Evacuated Tubes can reach a higher temperature (which is transferred to the water storage). But don't directly link temperature with energy. Temperature is the average speed of molecules and is measured on an arbitrary scale (Kelvin is the best one to use). Energy is the sum of the forces required to accelerate a mass a distance, it if more to do with storage in this case. So a raising the temperature of a large amount of water by a little bit can have the same energy as raising a little amount of water by a large temperature difference (the physics are simple of this one thankfully and water can store a lot of energy for little temperature rise).

The rest is down to the looks (I generally prefer Flate Plate collectors), the easy of fitting, the space available, the price etc.

You say you can't have PV, why is this? PV can often give superior performance (lower light levels) for any given roof area (though may not be large enough anyway).

The main thing to remember with all 'renewable' energy sources is that they are all give little energy when measured against land area.

Flat plate (FP) collectors will generally absorb more heat per square metre than evacuated tubes (ET). However, as soon as the collector fluid (generally a water/glycol mix) gets warmer than the ambient air both will start losing some of that absorbed heat. FPs will lose more so when the temperature difference is large (heating the DHW to hotter temperatures rather than just to UFH temperatures, say). The loses tend to be fixed for each type for a given temperature difference: ie., so many watts per panel or whatever. When the sunshine is very bright this loss doesn't matter too much but in weaker conditions it can rapidly become a very large proportion of the incoming energy.

Therefore, FP tends to produce more energy per year than ET but ET does useful work for a larger proportion of the year.

If you've got another source of heat and only a small area to use (eg., for aesthetic reasons) then FP will probably save you the most. If your other source(s) of heat are more expensive or more of a pain to operate then ET will avoid their use for more of the year.

What Steamy says about PV is strictly true: there are conditions when a given small area of PV will beat the same area of FP or ET in that the PV will produce some output when the thermal panels can't produce any. However, to get the equivalent amount of energy over a year you'd generally need a significantly larger area of PV than either of the solar thermal types, though the costs might be similar (as PV is generally cheaper but lower power per square metre).

In bright sunshine with low water temperatures solar thermal is cheaper per watt of power into the water than PV but not by a huge amount. In less bright conditions and with higher temperatures the PV is cheaper if you've got the space for it.

(Edited to add: cross posted with Tom but I think I answer what he said, anyway.)

Succinct and detailed explanation Ed as always

Posted By: Ed DaviesTherefore, FP tends to produce more energy per year than ET but ET does useful work for a larger proportion of the year.

And this is why in Italy for you with your massive heating load, FPs win by a mile. Once the heating season starts your biomass burner is going to be producing vast amounts of heat compared to your DHW needs. The extra HW ETs produce in the heating season will be insignificant to you but during summer you have (for a given collector area) the ability to cater for any number of guests and in the just before and just after heating season, when in Italy you still get lots of sun, you can be confident you will rarely need to burn (or other back up) for DHW. Cost and durability/longevity wise FPs are better too IIRC. Hence, a no brainer as I mentioned on the previous thread. Next question is sizing......

Where are you, what alt, are you above the fog?

Posted By: Ed Daviessometimes thermal panels will produce nothing but an equal area of PV will produce something

There must be a crossover, when with increasing insolation ST begins to produce 'something' while PV produces more, and at some point they equalise even for same collector area, before ST races away to produce much more for same collector area. Any 'feel' of where that equalisation point lies? You could postulate panels optimised in orientation and angle for mid winter collection, and acceptability of 22C flow temp if 'wet'. About what month, moving away from mid winter, might the equalised point come? Guess?

Tom, this is a bit of an unfair thread drift so I've started another thread to respond: http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=12793

Posted By: Ed DaviesTom, this is a bit of an unfair thread drift so I've started another thread to respond:http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=12793" rel="nofollow" >http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=12793

Now if I'm honest that is why I said PV is not an option. Not that I'm not interested.