Street's Disciple is a concept album...it's like a work of art. You have to listen to it all the way through. The production on it is a little throwback, but the quality of the production is amazing. Fuck, I'd buy Street's Disciple just for "Just A Moment." That song is fuckin unreal. Nas has obviously matured in the past few years, so you're not gonna wanna listen to this if you are strictly into radio (pop) rap.
Urban Legend is loaded w/possible singles, catchy hooks, and guest appearances. Personally, T.I. is overrated. His flow is cool and all, production is dope, but he's just not as multi-dimensional as Nas. This album was thrown together to move units. But if you appreciate catchy songs more than a conceptual album, you'll like Urban Legend.
Bottom Line: Street's Disciple is a work of art; Urban Legend is a hit-or-miss list of catchy songs. Buy Street's Disciple.