Is the attack modifier of green-flame blade or booming blade based on your spellcasting ability, or your weapon attack modifier?
For example, if my high-Charisma, low-Strength sorcerer were to use green-flame blade and make an attack roll with a longsword, would the modifier be his high Charisma mod, his low Strength mod, or his/the DM's choice of either one?
The spell specifies no exception to the normal rules for melee weapon attacks, so the attack uses normal rules: you use the Strength modifier, or if you're using a Finesse weapon, optionally your Dexterity modifier. Using other ability modifiers may become possible through magic (eg. Shillelagh).
The spell calls for a melee weapon attack and thus, without it specifically calling for something else, follows the normal attack rules and not the spell attack rules. For a non-finesse weapon that will use your Strength mod plus a proficiency bonus if applicable.
