I actually bought a 64gb SanDisk SD card at Best Buy Store about 2 years ago for the price of a 32gb card, because they were out of stock on 32gb SD cards.. I didn't ask for advice because I generally know what brand I want before I buy. And my second choice has always been Lexar. A Class 10 SD card generally works well on lesser expensive to expensive cameras. I am fortunate to have a Best Buy Store within 20 miles of my residence. And have bought my Sony pocket camera from Best Buy. The young men and women who work there, many of them college students, do the best they can in advising customers. I don't knock them down. They are sellers first, and advisers second.

Pixel count is an important consideration. Some high end full frame cameras have 16-20mp, which gives them fast frame per second rates >10fps, and excellent ISO/low light performance - qualities important for sports, and other kinds of photography where this is helpful.

People who shoot wildlife and landscape with good lenses often want the highest number of pixels, and though for birds, a high frame rate is helpful, you can get outstanding images with a camera that only does 5 fps. But being able to crop a 36mp or 46mp image without a serious loss of image quality has it's merits.

Then there are the smaller APS-C, M4/3 and smaller sensor cameras. In each of these you will find compromise - the first would be low light performance. The second would be loss of sharpness due to diffraction. The third would be differences in depth of field. As the sensors get smaller you need to use wider lenses for similar angles of view, and that will negatively impact your ability to get really shallow depth of field. The M4/3 cameras have extremely fast lenses available, with maximum apertures between F.95 and F1.2 to mitigate the depth of field issue. On the plus side, M4/3 lenses are typically high acuity, contrasty lenses yielding excellent image quality in a small, lightweight package.

Some cameras, like Pentax and Olympus, offer a "super resolution mode" that shoots multiple images with a very slight sensor shift to enable processing in camera of a super resolution image. A 20mp camera is then capable of a 40 or 80 mp image. This is not magic as it can be done in post processing with images from any camera, but it's nice that it is internal. The downside is that it is useless for images of moving subjects.

If you like the D300, the obvious choice would be the D500. It will feel familiar in your hands, and with a 20 mp sensor, you have a nice balance of speed (10 fps) and good high ISO performance. If you want better quality and more cropping options, a D810 or a D850 would not have many compromises. Canon has similar offerings, being very competitive. I think Canon's lens standards are higher than Nikon's on the whole, but this would be splitting hairs because often the differences exist on paper and not in the field.