With Supernova (i have nova) you can play more paches at the same time (multitimbrality)... at least 8 propably more... in Nord it's only 4 parts... but Nord has better sound character (I have only tested it in a shop)... Supernova sounds cheezy but you can tweak those presets and make it sound better.. there is also effects (delay, reverb, flange etc...) for each pach in Supernova... you have to buy effect machine for Nord or you have to add effects after you have recorded your stuff to computer.
You should also check Access Virus B and some Waldorf synths.
Good synth review site --> www.vintagesynth.com