Victory Medal or SF Package?

Everything in this is wrong. First of all, definitely choose the SF pack. You already get more than enough medals daily (even just sitting there you get 105 a week), and it is better to unlock new things than simply speed your farming up.

First of all, the E-100 isn't actually all that great. Yes, it is way better than the Maus (primarily due to stats), and it is slightly better than the KV-6 (primarily due to stats), but TBH it's buffs are pretty trash. It has one good buff and that is damage reduction. That is admittedly incredibly helpful, just not as much as the Maus buffs. You're still gonna have a hard time slaughtering tanks with it, and your chances of landing a oneshot are still pretty low. It can maybe kill half a medium tank if Rommel is on it. And only idiots use heavy tanks to attack artillery. Heavy Artillery is business left to other units, while regular artillery is suicide. Just use your HTs for killing infantry, people!

Secondly, the MKF3 is probably the single best special force in the game. It is so incredibly OP, you could use DeGaulle on it and it still is great. First of all, a fourth range with the vision to use it is already a lot to unpack. You are actually carrying a literal radar station with you. Furthermore, combined with it's stellar defense stats, the fourth range is a godsend that will allow your Artillery to last actually forever. It has a 60% chance of attacking again, so assuming you get a 100% chance of killing the opponent (and considering it's attack stats, you likely will), you will average 2.5X damage. Every round. To put that into perspective, the attack bonus of the Katyusha is 50% to two enemies in their spots who are next to your initial enemy. That is maximum 2X damage. Finally, it allows your other units to do more damage by deleting enemy fortifications. Just don't attack people in coastal forts that you need and you should be good. 

So yeah, get the SF pack. It's just better.

I'm F2P, only got into GOG3 recently and haven't been grinding hard. So far I've purchased three generals, Guderian, Leeb and MacArthur, and with them alone I'm into 1944, still finding the going pretty easy, and have four and a half thousand medals saved up.

The moral of the story: you really don't need either, but if you're going to spend, you have all the medals you need. Spend on something that will help you in combat, either the SF pack or the armor/artillery marshal badge.