Megapixels is not an indicator of the camera's quality. The number of pixels should be used to determine how large you can print or crop an image file. Just because the camera has 24MP doesn't mean that the image quality is going to be any good. In fact, take a look at the Sony A7S II. This is a $2,600 camera with only 12MP. If you were to compare its image quality to that of a D3300, the Sony would blow the Nikon out of the water. Now, of course, if you were to blow the image up large, or crop the image, the file from the D3300 would hold up better.
While the amount of MP that a camera has is important to know, but the size of those pixels is even more important. Why did Sony only put 12MP into a $2,600 camera? Who would use that?!? Well, because there are only 12MP on the sensor, that allows Sony to make the pixels physically larger. The larger the pixel, the lower the noise at high ISO settings, and the broader the dynamic range. For this reason, the Sony A7S ii has the best ISO performance of almost any camera on the market. But this great performance comes at the expense of not being able to print very big or crop heavily. For those who commonly shoot at ISO 1600 or above, this is going to be a very good compromise.
You're probably looking at the D3300 because it's a Nikon, and you're probably looking at the 6D because it's a Canon. On the other hand, you're probably NOT looking at the K-70, K-3ii or the K-1 because they're made by Pentax. Ironically a lot of people will only focus on Canon and Nikon because those are the brands that the pros use. Well, if you're going to be buying a $4,000 - $6,500 camera body and spend $12,000 on lenses, then yeah, stick with those two brands. If not, then you're paying more and getting less. Both Canon and Nikon are very much aware of consumer's buying habits. They know people think that all they have to do is buy a Canon or Nikon and they're going to get better pictures. This, of course, is just marketing BS to the Nth degree.
When purchasing a DSLR, your main concern should be the level of control and technical capabilities that the camera body offers. The body's impact on image quality is strictly limited to ISO performance and dynamic range. What's far more important is the camera's ability to allow the photographer to express his or her vision. The speed of its focusing system will allow you to take more shots of moving subjects that are in focus. Not that the in-focus shots will be better, but the percentage of in-focus shots will be higher. Again, we're not talking about image quality, but whether the camera is capable of allowing you to get the shot.
There are no "bad" DSLRs on the market today. Every DSLR today is far more advanced than any professional camera used a decade or so ago, and certainly better than any film camera. By better I mean in terms of its capabilities not image quality.
The 6D is technically better than the D3300 because: i) it has a larger sensor with bigger pixels. ii) the full-frame sensor means that you will be using longer focal lengths which create a shallower depth of field. iii) because the 6D uses a FF sensor, your wide-angle lens options are much larger than that of the APS-C format. iv) Nikons are very bad at video, while Canon is significantly better. vi)6d will focus faster and more accurately. vii) the 6D shoots at a faster frame rate and has a larger internal buffer. viii) the 6D shoots 14-bit RAW files while the D3300 can only shoot 12-bit RAW files. This means that the 6D has 4x more tonality than the D3300, which means a broader dynamic range and the ability to open up shadows to show detail.
However good the 6D is, and it's a good one, the Pentax K-1 one is much better. Like all Pentax DSLRs, the K1 has a weather and dust-proof body that is better sealed than Canon's 5D Mark IV. Like all Pentax DSLRs, the K1 has in-body image stabilization that is vastly superior to anything put out by Canon or Nikon. IBIS means that the sensor moves to stabilize the image. This means any lens you use will be stabilized as opposed to only specific lenses on canon and nikon. It also means less lens elements. C/N have to use extra lens elements in their IS/VR system which degrades the image. Pentax controls all 5 axis while in-lens solutions can only control 3.
For these and other reasons, the DSLRs by Pentax are reviewed as being better than the equivalent Nikon or Canon bodies. Where Canon and Nikon totally destroy Pentax is in the professional market.
So buy the camera that allows you to get the shots that you want to capture. Buy better lenses for better image quality. A pro body with a low-end lens will produce lower image quality than a cheap DSLR with a great lens.