Neither. Both are over used because though both are balanced characters everyone plays them, thus everyone can predict what's coming. Though I don't recommend it I learned to use Bowser, even though he is freakishly slow he taught me to react faster. Few can beat me with what many people consider a lousy character because few know what to expect from him and he is strong. I would recommend a lesser used character. There are many "lame" characters that have a significant flaw (Bowsers big and slow, mario has a small reach, etc) but if you can learn a character and learn to get over such weaknesses you can gain a significant advantage over the majority of players.
Theyre practically the same, but ike is based on power while marth is based on speed and each lack what the other has. So these 2 characters are the same, just placed on different sides of the spectra for different types of players.
In my opinion, Marth is better than Ike because, although Ike is stronger, Marth appears to be faster. Marth is also more agile and he can even pull combos off much faster than Ike. Another thing is that Marth's final smash is better since it's an OHKO.