The question is irrelevant because it assumes that Petr is 'owned' as if he were some inanimate object. We don't get to sell him to whoever we want because his desires and decisions are a huge, possibly the biggest, factor in where he ends up. As to why he would want to move to Arsenal over Tottenham, firstly obviously there has been no interest from Tottenham that we know of, but in a hypothetical situation where there is an interest from spurs, I imagine he'd still choose Arsenal because he's still world class and would want to play in the CL. All that said, I don't see how moving to Tottenham would be better from our perspective than moving to Arsenal.