Why Not Both? 

Install your car seat using the lower LATCH anchors OR the vehicle seatbelt, not both!
Only 3 exceptions currently exist in the USA:
-Nuna Pipa (all models)
-Clek Foonf (only when forward facing)
-BabyArk
If it’s not one of those three, use one or the other!
Reminders:
-Follow the weight limit for using the lower LATCH anchors on your seat
-If your seat is forward facing, DO use the top tether strap with either installation method.
Read and follow your car seat’s manual!