The S2 is a steel boiler and the Buderus is Cast iron. That difference itself can and does lead to end of the world discussions in the heating trade. I had a gas fired Buderus in my MA home and it was outstanding. I have worked with many oil dealers who swear by the S2. They do have excellent proprietary controls and are US made. The Buderus is a German boiler. Buderus throws more cast iron than all other boiler manuf combined and their cast is excellent. Buderus has their own reset control package. I prefer CI, possibly because I'm old enough that I think that is the way it is supposed to be

On these non-condensing boilers you have to take care of your return water temps and special attention must be paid to the vent system. The founder of S2 used to always stop by my booth at the trade shows back n the early 90's looking at what is new in venting as a vent system that is not "just right" will get torn up. The same goes for any mid-efficiency appliance. When they are riding that ragged edge of condensation you can have problems. As a reference, Appendix E in NFPA 31, Oil Installation, is an excellent guideline on sizing oil flues. It is an appendix because the 31 Committee (which I was a member of for 15 yrs and a dissenting member on this topic) did not want to mandate re-lining oil flues, as the gas industry had done. They feared that a SS liner would make oil non-competitive. Whatever you buy, and either will do nicely, remember that you are looking at a system, from the oil fill cap to the baseboard/radiant, to the vent termination. Look at all of it closely and make sure you installer does the same!!!! I think you will be well served by either appliance.