Both great options, stayed at both, but personally I prefer the Imperial or Taj purely for great F&B options in the properties. Some of those restaurants, (especially true in the Taj,) are definitely worth it by themselves! Note that Taj Palace club rooms are much more modern and recently renovated compared to Taj Mahal ones. Look into virtuoso if booking any of these.

Location can also be a factor. Leela and Taj Palace are more in the diplomatic area in the city center. (Wider, less congested roads, lots of roundabouts instead of traffic lights!) Taj Mahal too is very central near India Gate. The Oberoi slightly to the south.