The difference is in the input levels and power-supply voltage - HCT are designed to interface to TTL levels (0.8V / 2.0V). In general HC are the ones to go for if interfacing to other CMOS chips (they take 2 to 6V so are voltage-agnostic). HCT will in general work fine too if your power supply is 5V. If you want a simple 3.3V to 5V level converter you can use an HCT buffer powered at 5V - its input levels of 0.8/2.0V should be happy with 3V3 drive.