The Gamecube D Terminal cable much more rarer as the D-terminal connections are rare outside of Japan.
The Component Cable version is more in demand, as more equipment uses RCA connectors instead of D-terminal connectors.
Price wise, they both run $150 to $250 (its mostly sellers price gouging their customers thinking the market will bear).
Both cables can take the same Mod to Support RGB output (either as a 15 Hz RGB for Scart or BNC connectors or 31 hz for VGA)
They send the same electrical signal, it is just the plug is different. Which is fine my XRGB 2 Plus takes a D-terminal connection (and I don't have to waste with a D-terminal to RCA Component adapter).