Hello,
I have a 3090ti that I can take back and get a 4080. The 3090ti has 24gigs of VRam. The 4080 is faster but only has 16gigs of VRAM.
I do not know how big of scenes I will use. I will use Blender and other 3D apps to make short films. I want a 4090 but hard to get.
Should I get the 4080 or keep the 3090ti since it has 24gigs of VRAM and large scenes can fit into it?
Welcome to BA 
It’s hard to say how much VRAM you need. If you use Eevee, I’d stick with the 3090ti, Cycles, the 4090 will be better for sure
Hello,
What is Eevee?
I guess you should inform yourself about the different render engines used by blender and their pros and cons and then decide what hardware you need… because the hard truth is: any answer given here wouldn’t meet you needs then. This is better than just following any “advice” of any unkown person from the internet… 
You have an excellent GPU with a 3090ti able to make short films.
Unless you want to lower energy consumption, there is no solid reason to switch to a newer card, if you have not already try to make one short movie with it.
If you can have a 4080 for free, that is not a bad deal to obtain a recent powerful card that will last longer.
That takes time to learn to make one short movie, you will probably not use a graphic card optimally, at its full potential, during your first tries.
My advise is to save your money and learn to use Blender, first.
When you will have a more precise idea of your skills, how you want to proceed ; you could ask yourself about the perfect hardware necessary to do it.
That can take months to learn basics about how to animate, how to shade your scene, how to render.
And that can take months or years to realize a personal project.
So, months and years can pass before comes the time to make profitable your investment in a new card, that may not be so new, at that time or that could become less costly.
The 3090ti has 24gigs of VRam. The 4080 is faster but only has 16gigs of VRAM.
You should consider what is better:
So either get the 4080 and render with it or if too big of a scene, just let the cpu do the job?
I could get the 4080 buying adding on 250 dollars to the money I would get back from the 3090ti. But if the 3090ti is good and then maybe keep it until get a 4090.
It depend what you will render. If huge scenes with lot of 8K textures buy more VRAM to avoid “out of memory” and rendering with CPU. If smaller scenes and animations buy more speed. You can also wait for quantum computers and render farms based on them 
For Eevee, 24Gb VRAM is more important than GPU power. For Cycles I would choose 4080.
What would you consider a huge scene and what would you consider small?
Is one better than the other? What if I use Octane Render?
By huge scene in this case i mean a scene taking all 16GB of 4080 VRAM and by small scene I mean scene taking less than that. 16GB is a lot for me but maybe you are using lot of polygons with big textures and this will be not enough.
I am going to do stuff like Action scenes, Mecha, space flight, flight in atmosphere, Mountains, stuff like Gundam, Space shows, etc. Fantasy(Knights, dragons)short films, etc.
Then check how much memory your scenes takes now. For animations you need speed because small difference in rendering time multiplied by large number of frames can make big difference. Just chill. Life will go on no matter which one you choose.
I am new to this so do not know how much memory my scenes will take.
It depends on the scene - polygons, modifiers, nodes, light count, volumetrics, texture size, all contribute to VRAM usage. Generally speaking, you can get away with a few dozen lights and a few million polygons with 8 GB of VRAM. Or, you could probably fit a handful of 8K textures on a single 200k polygon character with volumetric lighting in the same amount of space. VRAM optimization is nuanced and multi-faceted. One thing that’s just good practice generally is not to use textures larger than 4096x4096. This will help a lot with VRAM usage
Oh ok so it depends on how much I do.
Depends what you want. Eevee is biased rendering, Cycles and Octane are unbiased.
Photoreal output benefits unbiased rendering.
Go big or go home. Go for the 4090.
I built a rig in Nov and settled for a 3080ti because I am in the early stages of learning Blender and UE and don’t need that much power “at the moment”. By the time I’m up to High poly counts, I’ll need the 4090 and hopefully the prices will stabilize. The rest of my rig was build for a 4090. 13th gen CPU, Z790 chipset, 1000W PS, 64 Gig Ram.
HOWEVER!!! before you get a 4090, read up about the problems with “melting” connectors.