The second problem of soul weapon SL is the fact that unlike the others 9, you will change them depending of the character you will use. It's the most flexible weapon, and for that, the one which will be the most likely "wasted" when you don't use the proper character (so when you use solomon/hercules/shingen who is not upgraded), while a SR weapon will be active all the time.
But yeah, it's situational. If you have a ton of SSR weapons, the soul weapon will most likely regain its #1 position in priority due to an higher amount of assault % and the fact you will need twice the amount of R to upgrade.
For damage:
The elemental advantage is only multiplicative to everything else in the full burst formulae (and double dip in this occasion).Damage = ATK on edit Screen
× (1 + Eidolon Character ATK↑ + Assault weapon + ATK bufs + Assist)
× (1 + Eidolon Elemental ATK↑ + Soul Weapon + Elemental Buffs + Elemental Advantage)
× (1 + Vigor Weapon + Vigor Buff)
× (1 + Critical)
× (1 + other buffs (rampaging, dmg vs stun, etc))
× (1 + Union Guardian)
× (1 - ATK Debuffs)
÷ {Enemy DEF × (1 + enemy DEF buffs)}