Let's not forget that Sculptris is free, too, much like the student version of Mudbox, and I'm pretty sure that would be another idea to learn that, as that would be the free version of Zbrush, more or less. Either way, student version of Mudbox or Sculptris, you can't lose. They're both worth learning.
PC Specs: Antec 900 ATX Case, Gigabyte 790GX Mobo, AMD Phenom II x4 955, Corsair HX620W PSU, Corsair Vengeance 16GB (4x4GB) DDR3-1600, EVGA GTX 680 2GB GDDR5, 3D Vision Kit 2, LG 8x Blu-ray Burner, Intel X25-M SSD, 500GB+640GB WD Caviar Blacks, Win 7 Pro x64, DAS Pro K/B, MX518 Mouse, Logitech Z-2300 Speakers, Toshiba 32TL515U 32" 3DTV, Wacom Bamboo Fun Large Tablet.