Is there a significant difference in image quality between an entry and a professional level dSLR?

How would a Nikon D700 compare with a Nikon D5600 in terms of image quality when the Nikon D5600 has double the MP?

If you are considering the D5600 as the entry level camera, you must then compare it to the Nikon D810 full frame camera. Both have the most recent technology.

In 2008, the Nikon D700 was an excellent camera, usually used as a backup camera to the Nikon D3s. Now 9 years later, it is a little "long in the tooth"

Usually "double" the sensor count on a camera can be significant, all things being equal. The true full frame Nikon digital SLR is the D810 and it has 36 pixels on its sensor

My guess is that you are comparing the D5600 to the D700 based solely on their current costs

The Nikon D700 is a full-frame camera. Therefore it has a much larger sensor. Larger sensor means having to use longer lenses which produce shallower depth of field which means better blurred backgrounds.

The D700 with its larger sensor also means that the pixels are larger. Larger pixels means better dynamic range and less noise.

Having a full-frame sensor also means that you can now use better or more lenses. Nikon does not make high-quality DX lenses, and they do not have a lot of options in the fisheye, ultra-wide, and wide-angle range of lenses. Using the D700 would open the doors allowing you to use those lenses to capture images in ways otherwise not possible with any DX camera.

According to DxO Mark, no DX sensor has ever been rated higher than an FX sensor.

Here's a link to dpreview.com showing the ISO performance of the D700 vs the D5100 and D7100: https://www.dpreview.com/...2720779485 Even though the D7100 is better than the D5600, the D700 still outperforms it at high ISO settings. The ISO performance of the D700 in the ISO comparison is higher because the sensor only has 12MP which allowed Sony to produce a sensor with larger pixels. Larger pixels produce less noise with broader dynamic range. Note, that this link can't be used to compare sharpness as that is based on the lens. Put a great lens on a D3000 and it'll produce better sharpness than a D5 with a bad lens.

You should only consider higher MP cameras if, and only if, you need to make prints larger than what 12MP would allow, or if you need to heavily crop your images. More than likely these won't be issues, and the features and performance of the D700, not to mention the versatility of shooting full frame, will far outweigh anything that any D5xxx series camera can offer.

If you know exactly what you are doing? Yes. The more expensive camera makes shooting easier allowing you to be more creative hence improving your shots. If not, you won't see the difference.

MP is only picture size. It has nothing to do with image quality.