Warranty is very important, as many of these TVs can die after a few years. Some brands, such as Sony, give 3 years while others, like LG, only one year.
The when it comes to OLED vs LCD you should know that OLED performs better in dark environments and has the drawback of a shorter life time. While LCD with full array backlight come very close to OLED an can be viewed in brighter rooms.