The top one is a (quite rare now) R32 GTS-t - not Type M - with optional factory alloys. The brakes on the black GTS-t are rubbish, especially for a turbo car. For this matter alone I'd go for the Silver GTS-t Type M (I've got one - see sig).
BUT - a couple of things:
**The Black car looks like it's in overall better condition. This is something you should really take into consideration, as R32's are getting long in the tooth now.
**The Silver car has the extra Type M goodies - 4-pot brakes, viscous LSD, GT-R style interior etc - but as with 99% of Type M's... it looks as though it's had a hard life. Expect to replace clutch, LSD, suspesion components; fix rusted out rear wheel arches (around the body kit), and other minor mechanical things.
Having said that, if you want to play it safe, go for the black car. If you don't mind fixing lots of stuff (but end up with a better car), go for the silver car.