There is definitely a difference between them. LED is the same as LCD by the way. 4K TV's look awesome with 4K content but since they are LCD, some still have LCD issues. OLED is superior overall but are very pricey at this time. But since you are looking for a 70-80" TV you won't find any 4K or OLEDs that size. I believe the biggest 4K is 65". If you must have that size you are only stuck with SHARP or Vizio which can't compete with the 4K/OLED/Plasma sets. I think it's time for you to upgrade for sure, good luck.
Media room- LG 4K OLED EG9600, Pioneer Elite VSX-90, Samsung 4K K8500, Klipsch Gallery G28's, Klipsch Sub 12, Klipsch RS-52 II, Optik HD cable
Living room- Sony 4K 79XBRX900B, Onkyo RC-360,HTPC, Klipsch Reference R-62II, Rc52II,PS4
Gaming room -Panasonic ST60, PS3, Wii