For those of you who can't see the Steam pages, they provide a little more depth:
OVERKILL PACK:
Get ready to rock!
The OVERKILL Pack is our 17th DLC for PAYDAY 2. It allows heisters to use the Minigun and the Rocket Launcher weapons, the OVERKILL mask as well as a new OVERKILL Escape Van skin. There will be 3 weapon modifications for the Minigun as well as a total 10 extra achievements added. The OVERKILL Pack adds a large amount of Hype Fuel for the Hype Train event with a chance of unlocking additional content for all PAYDAY 2 owners.
Key Features
New Minigun weapon – The Minigun is a 7.62x51 mm, six-barreled machine gun. The rate of fire is between 2,000 to 6,000 rounds per minute which is crazy. It has Gatling-style rotating barrels with an electric motor as its power source. The "Mini" in the name is in comparison to designs that use a similar firing mechanism but larger shells, which is funny, because the Minigun isn't that mini in reality.
3 Minigun weapon mods – Enjoy customizing the minigun with some really cool weapon modifications.
New Rocket Launcher weapon – The Rocket Launcher is a portable, unguided, shoulder-launched, anti-tank rocket-propelled grenade launcher. Because of its ruggedness, simplicity, low cost, and effectiveness it has become the most widely used anti-armor weapon in the world.
New Escape Van skin – Modify the classic PAYDAY gang Escape Van with a completely new and unique skin. Be the envy of all other heisters out there using this beauty.
1 new OVERKILL mask – A completely new mask that we call the OVERKILL mask is given to anyone who purchases the OVERKILL pack.
10 new achievements – An additional 10 achievements are added to give you more challenges to complete with your crew.
Additional Hype Fuel for the Hype Train event – Help the rest of the community gain Hype Fuel by purchashing the OVERKILL Pack. Buy the 4-pack and give the extra copies away to your friends. The Hype Fuel event is a community event between Februrary 5 and March 12 where the community work together in order to unlock free updates for everyone.
Bonus content with the Spring Break event – When the Hype Train event has concluded, all of the destinations that have been reached will generate free content that will be unlocked during the Spring Break event between 13-21 March.
TOTALLY OVERKILL PACK:*
This is totally OVERKILL!
The Completely OVERKILL Pack is our 18th DLC for PAYDAY 2 and is available for a limited time only. This pack is for anyone who wants a way to support OVERKILL in developing the free Hype Train content for the community. The pack allows heisters to use four self illuminating versions of the original PAYDAY masks. At some point in the future, a secret reward will be unlocked for all owners of the pack as well. The Completely OVERKILL Pack adds a huge amount of Hype Fuel for the Hype Train event with a chance of unlocking additional content for all PAYDAY 2 owners.
Key Features
Four special masks – We asked our artists to take the four original masks and make them OVERKILL. The result? Sheer awesomeness. If you love the four original masks, wait until you try these out.
A secret reward – For those who want to support us, we have a special in-game reward that we'll release at a later date - when the time is right.
Additional Hype Fuel for the Hype Train event – Help the rest of the community gain Hype Fuel by purchashing the Completely OVERKILL Pack. Buy the 4-pack and give the extra copies away to your friends. The Hype Fuel event is a community event between Februrary 5 and March 12 where the community work together in order to unlock free updates for everyone.
Bonus content with the Spring Break event – When the Hype Train event has concluded, all of the destinations that have been reached will generate free content that will be unlocked during the Spring Break event between 13-21 March.
Limited Edition – This DLC will only be available during the Hypetrain event. After the event is over, you won't be able to purchase it anymore so make sure to get it while you can.
TL;DR - Pack A gives you a lot of stuff, Pack B is more expensive and mysterious but is limited time only. Also, I'm guessing that buying the more expensive one nets you the cheaper one too.