Coupon vs YTM
Coupon rate is what the bond pays you in cash; YTM is your total return if you hold to maturity, assuming all coupons are reinvested at the same rate.
Two bonds with the same coupon can have very different YTMs if their prices differ. Always look at YTM, not coupon, when comparing.
The reinvestment assumption
YTM assumes you reinvest coupons at the same rate — in practice, reinvestment risk means your realized return can differ.
Zero-coupon bonds sidestep this assumption entirely, which is one reason they look attractive in falling-rate environments.