Which construction is better for an oil-fired home heating boiler's
performance and longevity, steel or cast iron?

Greetings Myheck,
Based on what I found while looking around at websites about oil-fired
boilers, I conclude cast iron is the way to go. Cast iron boilers are
a bit more expensive at the onset, but they conduct heat better than
steel (this is proven in sizzling plates, which are made of cast
iron), are stronger, last longer (for residential level boilers), and
need usually less maintenance than steel boilers.

Sources:

Utica Boilers - Cast iron boilers
http://www.uticaboilers.com/bc.asp

Dykeman Family Heating, Air Conditioning & Boiler
Corporation--Furnaces
http://www.dykemanheating.com/boilers2.php

Boiler FAQs - Quietside Boilers
http://www.quietside.com/pages/boiler_faqs.html

Trade Article by Tom Morgan "Heating System Retrofits" published in
The Journal of Light Construction 1988
http://www.soundhomeinspections.com/article.html

http://www.pmengineer.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2732,70941,00.html
- Shows that residential cast iron boilers last longer than steel, but
at the commercial level, stainless steel takes over.

SIME - Oil Fired Cast Iron Boilers
http://www.masterheat.com/ma02003.htm

Clean Burn Cast Iron Boilers - Sandri Sunoco
http://www.sandrisunoco.com/Cleanburn/castironboilers.htm


Google Search words:
oil boiler cast iron steel advantage
oil boiler cast iron steel better 
oil boiler cast iron versus steel

I hope this answer has helped you in the way you need. If you have any
problem with it, do please post a Request for Clarification and I
shall respond as soon as I can. Thank you.