With plasma, it depends on the roll. I got a good roll with my devastator plasma cannon and it took out the tank where the lascannon didnt.I suppose the title really says it all...I have a demolisher kit which I was initially going to assemble as an executioner with plasma sponsons for 5 plasma blasts per turn (4 if moving), but it occurred to me that multimeltas might be a good choice because while the plasma will devastate heavy infantry and light vehicles, it's fairly mediocre against tanks (it would probably not be my only tank in an army, mind, and a regular LRBT with hull lascannon and multimelta sponsons would probably take up the slack). Any guard veterans out there who can advise a newcomer?
I'd go with more shots. Use plasma. It will help...a lot.