wood beam vs steel beam

I'd agree with consulting an engineer on this one, but will just add that in the meantime, if it's sagging already, you should probably put something, even temporary, underneath that to help hold it up until a more correct solution is in place -- wedge the right size something underneath it (vertically) as a support where it is sagging...
 
You just need some to do the calcs on your spans and loads and they can tell you what size/type beams to use.

Guessing is not good.
 
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