Given that the Panasonic has 720P/1080i upscaling, then yes you should get a better picture from your TV when using HDMI, assuming the TV is at least HD-ready.
No, you won't need the Scart connection if you have an HDMI connection, but you should retain the RF coaxial link as this passes through a signal from your TV aerial to the Samsung, allowing you to watch broadcast TV even when you're not using the Panasonic.