Updated Answer: There is a HUGE difference in quality. 300 thread count feels like sandpaper compared to 1500 thread count sheets. The higher the thread count, the longer lasting and more luxerious the sheet. Thread count is how many threads you will find in an inch of fabric - the more thread, the thicker the sheet. The thicker the sheet, the more sturdy and longer lasting. And, the feel is amazing - super soft and luxerious. You know those exclusive hotels that charge $5000 a night? They use 1500 thread count sheets on their beds.
There isn't much difference in quality with a thread count over 300. You would next look to the material that the sheet is made out of. Pima cotton may be one of the softest and most durable.