I've actually begun to wonder how much better it'd be to have them both. Sometimes I've had to use my Minnie in the last event tiles to clear out an enemy deathball when I've wanted to save her for the enemy commander at the end. I've saved enough cores to buy one of the event girls in the exchange shop...but if I got PT-76 in a Soviet tank pack I was going to save them since Minnie is marginally better than Katyusha. However, now I'm thinking it'd be great to have them both.
People say they don't like her because she's defensive, but I use her speed to save the high-offense tanks I deployed first as they get damaged. If she can keep them alive by taking over the front line, they make up for her relative lack of offense and the whole deathball remains intact. And no, a heavy won't do what she can do.
I have three 5* heavies sitting idle because I simply can't find a good use for them...they're too slow to use in a deathball and they don't put out enough damage to be able to make their greater defensive properties mean anything. Larissa is enough of a tank to fill that role.