Buderus is probably the biggest boiler manuf in the world. I had a natural gas cast iron unit in my last home and it was excellent. You are correct that you do not want oil...under any circumstances.

The primary options for your "modulating condensing" propane fired boiler (which is what you want) are Buderus, Triangle Tube, Vieesmann, Lochinvar, Rinnai, Weil McLain, Burnham. What is most important is to find a good local dealer who can install and service the equipment you select.

If you have access to natural gas, then the decision is easy. Natural gas is currently much cheaper than fuel oil or propane. It would be my choice if I were picking a fuel for a boiler.

thanks- after looing more into it I do not have access ot natural gas. I actually think the fracking thing is pretty awful so don't want natural gas. I think probably the way propane or heating oil is extracted would not make me happy either-- which is why I had thought pellets would be good for me. But now I see the pellet boiler is So much more expensive so I think I may go with the buderus propane

A lot depends on your location and electricity rates. Reveal your location (climate) and electricity rates. In addition to that, is this new construction? If not, what kind of heat do you have right now?