With your plan I'd definitely park at the first non-pay layby on the Conwy/capel curig side of Pen y gwryd.
This is handy for both the miner's track up to the Glyders (in fact there's a short cut path to the miner's track via a stile directly opposite the layby - not easy to find coming down though) and also for the newly refurbished footpath from PyG to PyP (so you no longer have to brave the traffic walking up the roadside).
I have camped at caseg fraith, but there are limited dry spots for tents (annd it's all very wet up there at the moment) so if someone else gets there first you can struggle to pitch. Spectacular sunrise views of Tryfan though.
Llyn y cwn has more and dryer pitch sites (but can feel like a holiday camp mid season when I've seen 10 or more tents on the same night.
edit: just noticed that this was last weekend and you've already been. Android seems selective in letting me see posts. Surprised you find more dry spots at cf than llyn y cwn though.
Edited: 24/04/2012 at 23:52