our old design rule in 3025 era for energy balance was
PPC's for range, 3 ML's for every PPC, and then
if you have excess heat but not much weight, more ML
if you have excess weight but not much heat, SRM's or LRM's depending on your preferred role.
If I have an extra ton, my choice selection typically goes like this.
Is my armor maxed?
Do I need some special gear like ECM, AMS, supercharger, that would aide my designed role?
Is the design tending to run HOT? (buy heatsinks if you have the room for them)
Is the design constantly running out of ammo?
if no to all the above, buy a medium laser (ERML if running clan)