Yes, the P indicates Plated Steel which is not applicable to the loonie and toonie.
The WP is a bit more sought after due to its uniqueness as being the only time WP was used but neither are particularly rare. According to Charlton the 2003P come from the regular, Oh! Canada! and Tiny Treasures sets mintages 94,126, 61,146 and 43,197 respectively and the 2003WP come from the Special Edition (new effigy) set mintage 71,142.
2003P 50c MS65 NC $10, MS66 NC $20, MS67 NC $25
2003WP 50c MS65 NC $15, MS66 NC $30, MS67 NC $40