I dunno about you but I'd rather use the Exorcist than the Multi-melta Immolator. Transport capability for 6 Sisters isn't much to cheer about and for 10 points more you get something that has twice the range and thrice the firepower.
I mean...have you actually TRIED to get close with a Multi-melta rhino? Getting within 24" is possible...getting within 12" of anything that matters.... It's impossible.....Ever yone but other sisters is treating the Immolator like a heavy tank and not IFV that it is.
With the Exorcist you at least get a fighting chance to get off a good effective salvo. ESPECIALLY against other things that has thin armor(and preferably operate in squads). It's DAM effective against enemy Warbuggies, Sentinels, Vypers and that stuff.