Seconded... converting all the damage to acid makes it an acid spell rather than a fire spell. At least, that's how I would rule it. Some enterprising rules lawyers might note that the feat description says nothing about changing the descriptors of the spell, but I tend to cast Harm and Quickened Inflict Critical Wounds on such fools!
Of course, the drawback of the 50-50 option is that it makes the spell weaker against enemies with either resistance, and even more so against those who resist both! On the other hand, it also means it will do SOME damage if they are resistant/immune to one of them, so a fire/acid combo is a good thing to try to kill a troll with, for instance.