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

I'll assume you're talking about 'glue logic' and you're not trying to build a CPU out of logic chips.

I think you're better off using popular types of HC CMOS chips since they'll likely still be available in some years. 4000-series CMOS is too slow when operated from low voltage, and LS TTL is becoming rare, has lower fan-in and requires that you use HCT CMOS if you want to mix families because the output swing is not compatible with HC CMOS.

You may not care about power consumption, but most of us do, and if we're not designing in LS TTL, it will disappear sooner than otherwise.

Anyway, if you use 74HC CMOS you can probably find pin-compatible parts in CMOS, LS TTL and more modern logic families such as VHC CMOS. You can constrain your design to allow the use of any of them. For example, a part that might be used is a ' 573 latch. You could allow for a 74LS573, 74HC573, 74VHC573, 74HCT573 and probably others. All will operate from 5V.

You can look at the distributor stock and get an idea of what parts are on their way out. If they've not made the transition to RoHS and/or SMT you know they're dead chips walking, and if the stock at disties other than the ones that specialize in obsolete parts is low or nonexistent, they're probably as good as gone.

My favourite family for 5V glue logic is HCT for one simple reason, compatibility.

You can feed it's inputs quite happily from 5V TTL, 5V CMOS or 3.3V CMOS and it's output has both the drive strength to drive TTL parts and the voltage levels to drive 5V traditional CMOS parts.