Circumcision. There is no evidence that circumcision reduces sexual sensation. If this is a problem then it could be psychological and a call to your doctor would be in order.
Definitely. Foreskin covering glans prevents penis from rubbing against underpants. Penis slowly becomes less sensitive. Foreskin is very sensitive with many sensory nerve endings. Circumcision is advocated by some as cure for premature ejaculations because sensation is reduced. Recent danish study had good evidence for uncircumcised penises being more sensate than circumcised ones.