I have actually been down this path before, as my goal was to make 3D parody animations, something both tools are capable of doing. However, like everyone else is saying, they are completely different tools. Don’t get me wrong, they are both amazing tools which do really cool things for their users, but hopefully I can clarify some of the differences specifically in “Parody animations”.
The number one difference you will find between the two packages from a capability standpoint is the render. SFM is a cutscene creation tool built off a well-established game engine. It’s like Mario Maker in the world of Mario Games. You can get really creative, and the tool is really powerful, but you are limited to the art assets available. Likewise, SFM limits you to the engine’s rendering capabilities. It looks great for a lot of cartoony series, such as TF2, Mario, Sonic, and Splatoon to name a few, but if you wanted to go beyond that, you might find the tool somewhat limiting (more on that later). The other key difference is the target workflow. SFM pretty much replaced machinimas, because it natively accomplished the same goals in a much more straightfoward fashion. One thing it does is allow you to build a rough animation by playing the game. While you can do the in Blender, it isn’t as intuitive because Blender targets a wider audience than just gamers. Someone correct me if I am wrong, but I think SFM’s lip-syncing tool is more automated than even the best of Blender’s plugins, as it doesn’t require someone to type in the script. However, I think that’s the limit, whereas you can get much more expressive lip syncs manually in Blender. Lastly, game art assets are much more readily available in SFM. So if what you see done in SFM videos accomplishes the visual quality you desire, SFM might be a faster approach to getting your videos finished.
However, if you are looking to expand beyond those videos, as was my case, Blender gives you an open canvas to work on. It takes more effort to get started on a blank canvas than an environment where most of the assets are readily available and much of the work is already done, but Blender has the tools to make it worth it. In my case, I wanted wind physics to blow realistic grass and cause reflections in a character’s fur. That is a lot of calculations for a real time game engine, but Blender was developed expecting those heavy scenes. Additionally, I have more flexibility in Blender for the types of rigs I build. I built a rig with bones, that moved a softbody curve which moved the actual deforming bones. This allows me to mix physics and animation for “relaxed” movement, which I find intuitive. I don’t think SFM has that kind of flexibility. And of course there are many other tools in Blender too. Corrective smooth saved a poor weight painting job. I can build a city with hair particles. I can simulate a little girl’s dress as she twirls around next to her furry friend. I can morph appendages into swords and animate custom particles to a very specific behavior. And while SFM might be able to do some of these things, it can’t do everything. And I haven’t even started on Cycles materials and shaders or even Freestyle.
Hopefully that breaks things down enough for you. They are both amazing tools. SFM is designed for gamers to get quicker results, but is restricted by the game engine. Blender is a general purpose, more powerful tool that can achieve better results at the expense of time and energy. Both are amazing tools developed by amazing developers, and both achieve the goals of their target users. But like everyone else said, what a person should use ultimately depends on what they want to do and what quality they want to achieve.