If upgrading from a Rebel/***D series camera, or even an earlier **D model (like the EOS 60D), the Canon EOS 80D and EOS 5D Mark IV can each provide unique benefits that make them sensible upgrade candidates. So let's break down the differences to see which body provides the right upgrade path for you.
Let's first look at the EOS 80D as it will likely provide an easy, seamless transition for those who are already using a crop-sensor camera such as a Rebel/****D/***D/**D. By "seamless transition," I mean that all of your current lenses should be compatible with the 80D. I say "should" because there's a very small chance that some older third-party lenses may not be fully compatible with bodies released after their manufacture.
And make no mistake, compatibility with EF-S lenses can be a significant benefit. Lenses designed specifically for crop sensor cameras are generally smaller, lighter and less expenisve than their designed-for-full-frame counterparts. Of course, EF-S lenses do have their drawbacks, such as often a lower build quality and a lack of weather sealing.
Now let's look at the 80D's benefits over the 5D Mark IV:
Of the benefits listed above, the most compelling for most consumers is the significantly lower price. In fact, you could nearly purchase (3) EOS 80Ds for the price of a single 5D Mark IV at MSRP (without rebates).
That kind of price differential brings the 5D Mark IV's numerous benefits into perspective. And while we're on the subject, let's take a look at the 5D Mark IV's benefits over the 80D:
One benefit I did not list for the 80D is the FOVCF (Field of View Crop Factor). Because the 80D's sensor is smaller than the full frame sensor found in the 5D Mark IV, it captures a narrower angle of view at the same focal length compared to the 5D Mark IV. A good description of the effect can be found in our Field of View Crop Factor page:
Although the physical focal length of a lens is not actually changed on a FOVCF camera, the subject framing certainly is. By multiplying the lens focal length (or focal length range) by the FOVCF, you get the full-frame focal length lens subject framing equivalent when used at the same distance. For example, if you are looking for similar framing that a 50mm lens (the classic "normal" lens) provides on a full-frame (1.0x crop factor) SLR body, you probably want a 35mm lens on your 1.6x FOVCF body. 35mm x 1.6 = similar framing to a 56mm lens on a full-frame camera body. This focal length is often referred to as the "Effective Focal Length". The lens is still a 35mm lens, but your final image will only include a crop of the lens' complete image.However, the FOVCF is a double-edged sword. On the one hand, it provides a more narrow angle of view which can provide [seemingly] more reach to your telephoto lenses. But with the 5D Mark IV's extra resolution, you could argue that framing a subject more loosely with the same lens would provide better cropped-to-the-80D's-resolution image quality with the ability to optimally frame the subject post capture.
Where the 80D's FOVCF becomes especially problematic is wide angle photography. Because crop sensor cameras provide a narrower angle of view at the same focal length compared to full frame cameras, wide angle views are sacraficed when using full frame compatible lenses on the 80D.
A big advantage of the larger full frame sensor camera is the ability to create a stronger background blur. Because a longer focal length is required for the 5D IV to create the same subject framing as the APS-C format 80D, the background can be more diffusely blurred in comparison.
So which DSLR should you get between the two bodies compared above? As usual, one's personal preferences, specific needs and budget will provide the answer. That the 5D Mark IV is the more capable, better spec'd body is an easy conclusion. However, the price difference between the 80D and 5D Mark IV is substantial, and one must be able to justify the 5D IV's superset of features to justify the higher investment.
Those who may be easily able to justify the 5D IV's higher investment include photographers who primarily appreciate the camera's better image quality including cleaner high ISO results and higher resolution, increased shutter durability, dual memory card slots, wider angles of view and 4K recording capability. And for those photographers who don't feel that the 5D IV's benefits are worth the incremental price difference over the 80D can enjoy the wealth of features afforded by the crop sensor camera at roughly 1/3 the cost.