Mini Mag is the better of the two for me, Keith Calver's stuff is always interesting but to be honest I only ever buy it now if theres a specific article I particularly want to read at my leisure - usually I just skim read it in the newsagents and put it back on the shelf!
The best car mag for me since the bitterly missed CCC (Cars & Car Conversions) disappeared is Practical Classics which for me is better written, more interesting and much better valve.
the trouble with the dedicated Mini publications is theres only so many times you can read about concours Cooper S rebuilds or 1380 'fast Road' monsters before it gets boring.