Hoo boy... I'll start with the 500 pt. list first.
1. Kantor is too expensive for 500 points. He isn't durable, and his special rules go to waste. Just take a 100 point chappie or Librarian. No need to buy upgrades, you have a capable character, and you won't break the bank.
2. Drop Pods have the "Drop Pod Assault" rule, where half (rounded up) of your pods arrive first turn. So your Dread will have to drop first turn.
3. You need full 10 man Marine Tactical squads. You get free special/Heavy weapons, and 10 man either provides staying power or combat squads.
Here's a decent 500 point list I used in a tourney, won against everyone except the club champ. Even beat the cheese dealer with a Russ
Librarian w/ Avenger/Machine Curse
Tactical Squad: 10
SM, Melta gun, Plasma Cannon, Sarge with Powerfist
Tactical Squad: 10
SM, Melat gun, Missile Launcher, Sarge with Power Weapon and Melta bombs.
Comabt squads, Heavy weapons hold objectives, Special weapons and Sarges advance, Libby takes care of vehicles.
1200 points... That's wierd. Eh.
Your list is better, but it's still lacking. Why Kantor? Take a Captain with Relic Blade, Storm Shield, and Artificer Armor. Only a handful of points more, and you get 2+/3++ and
Str.6 attacks. Much better than Kantor's 3+/4++ and
Init 1.
PF.
PFs belong on Sarges anyhow.
Dreads will work well, but the Termies will fail like that.
DS leaves them open, and every
AP 2 and 1 weapon will go for them right then. You'll lose too many, as 5++ is near worthless.
Also, more troops! You're running them around, right? A smart opponent will just focus on them, and 20 Marines won't last in the open against a whole army's worth of shooting.
Ditch the Termies and Kantor. Take more troops, a Captain, and maybe another Dread. The skimmer might work, I haven't used them before.
Oh, yeah. Don't rely on Orb. Bombardment. It sounds great, but isn't a sure thing. It's gravy on a Master, not the reason to take one.