Need a good Wifi Router with decent range for around 4k

Help me decide for a wifi router

I shortlisted
Asus RT N13U
Belkin N600 DB
TP Link 1043ND

Torrent client features of Asus is good
TP link has external detachable antennas

Are internal antennas any better?

Info:
It need to cover about 1200 sq ft on two floors,
Budget is 4000, can extend a bit upto ~400 if it is worth it,

Sorry if these are stupid questions,
Any other suggestions welcome