Is N Gamer the magazine that's incorporated NGC? Before I stopped buying them I largely prefered NGC to ONM because of their humour and insight into Nintendo's history. ONM can get too official at times and in the past they've marked up review scores to try and make games seem better which makes the magazine unrealiable.
This being said, with the Wii using regular discs as their game medium ONM may start providing demo discs with their magazine once it's released which would be useful if you intend to buy the console.
I myself only buy GamesMaster now. As a multi-format magazine I find it gives me the all printed information I need; any other info I get from forums, Gamespot, IGN and GameCentral on Teletext. Unlike other multi-format magazines it doesn't include long-winded industry reports or go text heavy on reviews and previews, and provides a large array of pictures and varied news items.
In the end it's your choice and you should base it on which magazine you like the most. I honestly can't say that I've read either one recently enough to make a valid opinion and can only state what I've learned from them in the past.