I'm not really great at math but I made some computations and I think the only case when plasma does significantly higher damage is when enemy is wearing Metal Armor or Metal Armor Mark II. Anything else is a difference of about 1 HP or less. Correct me if I'm wrong:
Magneto: 20-35, average damage: 27.5
Plasma: 15-35, average damage: 25
1. No armor
- 0/0 DR/DT vs Magneto & Plasma
Magneto: 27.5 (2.5 HP more)
Plasma: 25
2. Combat Leather Jacket
- 20/0 vs Magneto
- 10/0 vs Plasma
Magneto: ( 27.5 * 0.8 ) = 22
Plasma: ( 25 * 0.9 ) = 22.5 (0.5 HP more)
3. Metal Armor Mk.II
- 80/2 vs Magneto
- 25/4 vs Plasma
Magneto: ( 27.5 - 2 ) * 0.2 = 5.1
Plasma: ( 25 - 4 ) * 0.75 = 15.75 (10.65 HP more)
4. Tesla Armor
- 90/6 vs Magneto
- 80/10 vs Plasma
Magneto : ( 27.5 - 6 ) * 0.1 = 2.15
Plasma : ( 25 - 10 ) * 0.2 = 3 (0.85 HP more)
5. Combat Armor Mark II
- 65/3 vs Magneto
- 50/5 vs Plasma
Magneto : ( 27.5 - 3 ) * 0.35 = 8.575
Plasma : ( 25 - 5 ) * 0.5 = 10 (1.425 HP more)
In the end, considering that plasma has a range shorter by 15 hexes (that's really a lot, do not underestimate range...) and most energy users use aimed shots, in my opinion it's just not worth to use it, even if it was cheaper...