Hey.. so I'm guessing isolation isn't a priority since you'll be using them mainly at home (iPad) so I would go with the Sennheisers.. They have way better soundstage than the M50s and I believe the bass on the HD 449s will satisfy your needs..
Beats by Dre usually increase bass by something like 20 dB, making it muddy and artificial.. If you want a good set of headphones, get the Audio Technica ATH M50s.. They're a bargain and the build quality is amazing.
-Easy to drive
-Excellent build quality/price
It's a great first purchase.. Also, try to get the ones with the coiled cable, they're much easier to carry around.