This is tricky because you can buy a used car with valid tax and continue to use that tax disc.
But a SORN is different:
A SORN declaration is valid for 12 months unless you re-license, sell, permanently export or scrap the vehicle before the 12 months have expired.
The DVLA also require you and the seller to inform then as soon as possible of the change of keeper.
In light of all this, it would be better to put the most recent date on the forms.
You may have agreed to buy the car in November and given a cash consideration for the contract, but the transaction was not completed until March.