There are lots of choices when it comes to home boilers - if you have an open mind, where should you begin? First of all perhaps you ought to decide what factors really matter to you.
You could think like this - there is an initial outlay, then there is the ongoing cost. On top of that there is the environmental concern, and then there is the need for the unit to do the job. Each of these will matter in differing amounts to different people.
If you want an environmentally friendly car you usually have to sacrifice something, usually performance. So you could run a tiny car but it will not cruise so comfortably nor accelerate so quickly. This is not the same with home heating boilers - you can get great performance, better performance even, from the more environmentally sound products. This is probably because there is not much of a market for a heating product that will not heat the home!
If you are environmentally concerned, then you should put plenty of effort into making the best use of the heating you put into your house. This is all about good insulation and so on. There are lots of things that can be improved in most homes.
When it comes to your out-of-pocket one-off cost verses the ongoing cost, you will usually find that the more expensive systems will run more cheaply. This saving may or may not turn out to be worthwhile financially, depending on your home, the depth of the winters and so on. Usually gas radiant heating, for example, costs more than an equivalent electrical system to install, but will work out much cheaper day in day out.
The trick is to select the system that matches your preferences.