I'm honestly curious as too which one is better, mostly because they both mount the same cannon but the Valdor is significantly more expensive and has worse BS.
The cheapest loadout for the Sicarian is 200 points, while the Valdor is 315.
They both have a Neutron laser Destroyer, while the Valdor carries an Autocannon or heavy bolter to the 2 heavy bolters of the Sicarian.
The Valdor's benefit is a higher T 8 vs 7, and more wounds but the Venator has a 2+ save compared to the Valdor's 3+.
I know they're forgeworld products but it seems unfitting that the IG Tank is that much worse than the Space Marine one.