Not sure about the exact panel in CX420,you can check the panel info in the service menu,during standby press i,5,vol+,power .
If you see something like LTY320HN0102 as found in EX520 where the first letters can be either LTY ,LTU,LTZ,LTA then its a SPVA rest of the numbers and alphabets after the first three letters are irrelevant.
The C420 has the older inferior BE3 image processing while still good looses out to the newer X-reality in CX520 and upwards.
Another good choice is the samsung D580 this one comes with a SPVA,