I was priveleged to be part of a judging panel that tested 10 different brand / model machines in all different price brackets a couple of weeks ago and the results were interesting....if for no other reason, it showed us (I think) that it is not so much a question of thermoblock VS boiler, but a question of comparing rather, the result (or "performance") obtained (ie coffee & steaming) from the "working end" of any individual model.
So what I am saying is, some boiler machines "produce" better than others, and some thermoblock machines "produce" better than others, and some boiler machines "produce" better than some thermoblock machines, and vice versa, and there are different pricing brackets to all!
Some machines just perform better than others...irrespective of their internals....
Having basically just said such nice things about thermoblock machines in general, I will personally remain biased against anything that has its guts made of aluminium (sorry, but aluminium, HEAT and water dont mix....atleast not in my stomach).
Notwithstanding, dont forget the other caveats such as the fact that in general, most thermoblock machines are "electrical appliance" brands and their long term repairability including availability if parts and / or availability of service agents is suspect compared to the fact that in general, most of the "better" and / or more expensive domestic machines branded or manufactured through traditional coffee machine channels are boiler machines and whilst this has nothing specifically to do with the thermoblock VS boiler thing, this group of machines has good long term repairability and availability of parts and repairer networks....so is still related to the discussion overall.