Cameras are so brilliant these days that megapixels and ISO capabilities aren't really an issue anymore.
Admittedly, they are considerations to take in. However, they shouldn't be a reason to buy a new camera. If you've already bought into a camera system, chances are you have a good idea of how the cameras work and feel. You also probably have a bunch of lenses for that system.
So this guide is more for those who are just starting out on their photographic journey and need a little guidance.
Below are 3 considerations when choosing a new camera:
The first consideration should be ergonomics. How does the camera feel in the hand, can you hold the grip properly, is the weight comfortable, are all the buttons accessible? If the ergonomics of the camera are perfect for you, it should be a no-brainer. You are going to be using the camera for thousands of hours into the future so you need to make sure the system you have is comfortable.
Secondly, the camera brand you're buying into. Are you buying into Nikon, Canon, Sony etc? And why? When you buy into a system, you're probably going to be in it for life so you want to make sure the gear offered by the brand can sustain your personal growth. Lenses and flashes will most likely be the first purchases you make. Canon and Nikon currently offer the widest range of Lenses and flashes. While there are third party options offered, they are not always of the same quality.
Lastly, what are your subjects going to be? There's no point buying an entry level DSLR if the plan is to shoot weddings or low light events. It really won't be up to the job. ISO and focus might be, but lack of changing settings on the fly will be the biggest problem. In an opposite situation, if photography is going to be a hobby that will be touched upon about once a month, an expensive flagship camera will also be pointless and overkill.