Mid-range digital single lens reflex cameras often use the same sensors as the entry-level cameras from the same manufacturer. The difference between entry- and mid-level models generally lies in the features set, not the light-gathering capabilities.
Additional features include:
Rugged body: mid-range DSLRs generally offer more rugged bodies made with a mix of plastic and metal. These bodies, while not being totally shock-proof, can withstand a little more shaking and bumping than entry-level models.
Weather sealing: while only one brand currently offers an entry-level weather sealed camera, many mid-range models offer some level of protection against the elements. The weakest link is often the lens, however, and a lens that is not weather sealed will be of little use even if the body is protected.
Larger body: Mid-range DSLRs have larger bodies than entry-level equivalents. While a larger body is more cumbersome, it offers more real estate for the placement of buttons and shortcuts. Users who enjoy changing settings often will find the use of a mid-level DSLR easier and more convenient. In addition, a larger body will feel more balanced when used with a large lens.
LCD display: Mid-level DSLRs often (but not always) offer larger LCDs, sometimes with better resolution. Some models also sport a secondary, monochrome LCD, which is used to display current camera settings. In addition to preserving battery life, such a display is extremely convenient for quickly reviewing camera settings.
Larger viewfinder: as mentioned earlier, the viewfinder of a mid-level DSLR is generally large and bright. A larger viewfinder has another advantage : the fact that more information can be shown on the viewfinder display.
AF points: generally speaking, a more advanced camera offers more autofocus points and the AF system is often more efficient and focuses faster in difficult situations.
Vertical grip option: many mid-range DSLRs can use accessories like a vertical grip. Such a grip is used for portrait oriented photography, and provides additional battery storage. Some also hold small accessories like remotes.
IR port: some mid-range models can be triggered by an infrared remote, allowing for easier operation in difficult situations (birds photography, self-portraits, etc).
Shooting options: mid-range cameras propose shooting options not found on most entry-level bodies : focus and exposure bracketing, multi-exposure, white balance fine-tuning, lens-by-lens autofocus compensation, etc.
Which type DSLR?
Only you can decide which level DSLR is best for you. Make a decision based on your current level of expertise and your current and future needs. Remember: the best camera is the one that you enjoy using, whatever its features list!