Onguma's a great little campsite. Thoroughly recommend it. They only have 6 (or 8) sites though, so book well ahead for Dec. There's a bar, restaurant, pool and they have their own waterhole too. Each campsite has a huge personal fire space, cooking/washing up space, power, a v clean bathroom in a rondavel - and they even bring complimentary fire wood around for you.
Outside Andersson Gate, Etosha Safari Camp has a nice patch of lawn, a bar, pool and restaurant too. It's about 9km south of Andersson Gate.
In my opinion... although the camps inside Etosha are ridiculously expensive now - you've obviously gone to a hell of a lot of effort to get yourself to Nthn Namibia and it'd be a shame to go all that way without staying for at least one night in a camp. Even if you see nothing at a waterhole - it's still a pretty cool experience sitting around the waterhole after all the day-trippers have gone home, with your beer/wine hoping something will show up, laughing at the jackals chasing moths and bragging about what you did/didn't see during the day with everyone else. And the night sounds are amazing - esp lions roaring which you won't hear if you're at an outside camp.
Depends how many days you want in Etosha, but if it's your 1st trip to Namibia, you really want to experience a night in Etosha. I'd possibly stay otuside at Onguma (as that camp is great) and then inside for a night at Okaukuejo. If you have more time after that - move out to Etosha Safari Camp to save some money.