I'll have to double-check, but the impression I get is that currently vanilla infinite research has a fixed cost, which would mean that the research would never switch to anything else. Then again, I've only reached 100SPM momentarily just recently, so perhaps it will switch between different research later on.
I had a look, I'm clearly wrong, but it doesn't seem to switch as yet; perhaps it was just researching what was cheapest all along, and I wasn't paying attention?