Let me answer you:
US SSes - I don't know. Never played SS or US nation.
UK SSes - Highest SD and repair rate. <<<--- DON'T underestimate this. It's easiest for UK to reach 280 rep/sec and 900SD. Because of this, UK SS game play can be quite different from other nations.
Also I do admit UK SS5 is the 'BEST' SS there ever was!