Darn good price for this TV. Only comparable model would be the
46D6000 LED from FDW @ R10,295. Depending on how much watching you're going to do, the reduced electricity usage over the TV's lifespan will probably make up the difference.
Then its down to the technology/feature differences between two models e.g.
51" glass screen vs 46" clear synthetic screen
LED brightness/pop in well light conditions vs Plasma natural look in darker conditions
FHD 3D on the D550 vs half FHD 3D on D6000
100Hz engine + LED motion vs 50Hz engine + 600Hz Plasma panel refresh
SmartTV on D6000 vs no smart features on D550
Best to check them out side-by-side if you can, to decide whats more appealing to you.