There is no cost involved, though, so giving them away would be fine - it's just an issue of whether or not to contribute to landfill.
Cost of 16GB Apple factory upgrade: $200
Total RAM: 16GB
Cost of 16GB 3rd party upgrade: $200
Total RAM: 24GB
The 24GB will be "enough" for a bit longer than the 16GB will so the opportunity cost of spending the remaining $200 (or maybe even less in the future) to do the final upgrade to 32GB will be lower for the second option as well since you can wait more time before you will feel the need to do it. Having useless 4GB chips when all is said and done involves no cost and is a fairly minor nuisance compared to the advantages. Also, you probably want to keep those chips for warranty repairs until your machine is out of warranty at least (and maybe longer if you plan to continue to use Apple service).
On the other hand, if your goal is to have 32GB very soon, then the best option is to do the factory upgrade to 16GB (to save the landfill) and then add 16GB 3rd party. It's still a $400 total expenditure (half to Apple, half to 3rd party), but why bother with wasting chips.
The other issue to consider is warranty service. If you ever have to send the machine in then you pretty much have to remove all 3rd party RAM. The least hassle of having to juggle static-sensitive chips in that case would be:
8GB:
- stock RAM
- total cost $0
- no swapping issues for repairs
- wasted chips only if you later go up to 32GB
16GB (option A):
- 8GB stock + 8GB aftermarket
- total cost $100 (cheapest day 0 cost for 16GB)
- have to remove second 8GB for repairs
- wasted chips (and money) for any upgrade beyond 16GB
16GB (option B):
- 16GB factory upgrade
- total cost $200 (same as next 24GB option)
- no swapping issues for repairs
- no wasted chips for future upgrades
24GB:
- 8GB stock + 16GB aftermarket
- total cost $200 (same as previous 16GB option)
- have to remove 16GB chips for repairs
- minor waste if you ever upgrade to 32GB
32GB:
- 16GB factory upgrade + 16GB aftermarket
- total cost $400 (cheaper than factory 32GB upgrade)
- have to remove second 16GB chips for repairs
- no wasted chips
I don't think I'd recommend anyone do the 16GB option B above since it is more expensive than option A if today's dollars matter to you and/or you think you will never need more than 16GB, and it leaves you with less memory than the 24GB option with little advantage other than not having to pull some chips for the off-chance of a repair. The only time it would make sense is if you are actually using it as an ordering configuration stepping stone to get to 32GB immediately or very soon as in the last option. If you are going to "hold off" on getting to 32GB for any appreciably amount of time, then the 24GB option offers you more time to "hold off" with only minor waste. And any time you can add to the "hold off" period just makes the eventual upgrade even cheaper (whether by price drops or by opportunity cost of holding on to the money).
