According to the GM media website, the 2008 Suburban 1500 was available with the L76 6.0L V8. This is different than the LY6 in the 2500. The other difference is that in 08 the 2500 got the 6 speed HD tranny while the 1500 got the 4 speed 4L70E tranny. You are most likely looking at an LTZ.
If you were talking a GMT800 platform and can get by on payload, the 1500 with the 6L and 4 speed would give better performance (acceleration/get up and go)then a 2500 with the 6L and HD 4 speed.
However, when comparing the GMT900 I would take the 2500 with the 6 speed tranny over a 1500 6L/4speed anyday.