The northern Carneddau are good but one issue is the lack of decent water sources high up - depending quite where you want to make camp you either need to be prepared to carry water up from lower down or else have a filter that you trust for taking water from the odd brackish pool up on the tops.
Of the Arans or Arenigs, both are good and both are relatively empty. My view is that the Arenigs are more likely to involve off-track navigation and larger boggy areas - so given what mattb has said he's after, I'd probably recommend the Arans...
There's good camping to be had near the small llyn north of Aran Benllyn, or else that llyn makes a good source to take your water and only have to carry it along the ridge to any other spot you pick. It should be possible to find numerous spots as the ridge meanders south to Aran Fawddwy. You'd probably also get pitched on the broad col just north of Drws Bach, with water available nearby and a nice view to the Rhinogs, but it is a bit tussocky.
As for routes, you can start from the north or south. From the north, there's free parking in a small car park by the bridge in Llanuwchllyn, and a straightforward (and not particularly long) ascent south along the ridge to the Arans summits. More problematic is the return unless you simply go out and back. You can descent easily SE from point 872 to Craiglyn Dyfi and Foel Hafod Fynydd, then to Bwlch Syrddyn (boggy) for the footpath out through Cwm Du, but depending what success you have finding the footpaths around Talardd you could be left with a few km on tarmac back to the car.
The approach from the south is probably a bit more satisfying. There's a car park with portaloo at the head of Cwm Cywarch, and a steady climb up the path on the south-east flank of Hengwm. Follow round to Drysgol and Drws Bach. Then it's out and back along the ridge to the Aran summits, taking your pick of the places to pitch. Return via the path SW off the summit of Aran Fawddwy as far as the col NE of Craig y Ffynnon. Then descend by the stream ,or else continue over Glasgwm and finally down the (initially) steep footpath just before Y Gribin.
You can do either of these Arans options in a day, but split over two days they give you plenty of time to linger and enjoy the superb location, and of course a grand night out that'll feel wild and remote and empty 