Should I use CMOS vs. TTL?
A few arguments:


with HC and HCT, the fan-out (at lower frequencies) is essentially unlimited, for LS it s IIRC 10 (or was it 20)?
the HC and HCT families are newer than LS, so my bet would be that they will be around longer than LS
HC and HCT use much less current than LS (except maybe for a few gates that switch at a very high frequency), which eases the design and layout. HC and HCT also require much less power supply decoupling
check the logic levels, and choose the best match: HC or HCT. LS gives you only one option.
last time I checked, there was more choice in HC/HCT than in LS


Let me add two (weak) arguments in favor of LS:


inputs can be left open (they will be high)
more robust against ESD