Hi! The main thing is switching voltage value. LS and HCT series are working like an ordinary TTL ("L" below 0.8, "H" from 2.4 up to VCC). HC serie is working like CMOS ("L" below 0.8V, "H" - I am not quite sure - about 90% of VDD). The next is power consumption. But I think you can find details using f.e. Google web site.
I prefer HCTL but sometimes I have been using HC instead of HCT. Sometimes I have been using LS instead of HCT. But all depends on your application.

Regards,
Jacek