Although both of these high-action movies share the same theme of one man hunting down criminals, they are actually quite different in style. 'Taken' is all about violent action and chases whereas 'Man on Fire' is more about redemption as well as the ruthless hunting and putting-down of a gang of kidnappers. Both films contain pretty extreme torture sequences and so aren't for the squeamish.
My personal preference was for 'Man on Fire' which takes much longer to turn into the hunt, but which contains much better drawn characters and genuine emotional connections and content. When the violence starts, it thus seems all the more shocking and believable.
'Taken' seems to have a few holes in the plot but is nonetheless a powerful film. Its characters are less rounded than in 'Man on Fire' and the plot is certainly less complex.
However, if you enjoy action thrillers which feature one man against the rest - then these films won't disappoint.