Unfortunatelly, neither. Main problem with his surname is the ‘ś’ (proper spelling is ‘Kościuszko’), which doesn’t appear in English at all. The closest equivalent I can think of is Japanese sound at the beginning of words such as ‘shinobi’, which I guess English-speaking people still pronounce like ‘sh’ in ‘fish’…

And then there’s ‘ci’, which is pronounced similar to ‘s’…

The closest I can get is ‘Koshch-oosh-kuh’, but that’s still not it (that would be spelled ‘Koszczuszko’).