More than difference of 2 years as if you include building period. Ec need 3 (building period) + 5 years (mop) while pc is only 3 years (ssd).
EC got 5 yr MOP but PC got 3 year Seller Stamp Duty (because no investor in the right mind will sell their PC within 3 years and pay SSD). So effective it is only about 2 yrs difference. The difference will be even narrower when EC sellers ask for exemption to their 5 yr MOP, hence right, EC and PC price will continue to narrow.
Resale prices will get hit again. So property buyers can expect their units to be sold cheaper and cheaper as the lease starts to run out.
Another thing is probably Govt is doing this to ensure sufficient demand for Tengah development. Expect lot of BTO and EC coming in that area for the next couple of years.