Hello, Sanjay.
Have you checked this comparison overview on ASR: (http://www.cisco.com/c/en/us/products/routers/asr-1000-series-aggregation-services-routers/models-comparison.html). The Cisco ASR 1006 can support more SPA interface compared to the ASR 1004. The Cisco ASR 1006 will be much scalable compared to the ASR 1004 as far as throughput.
What do you mean balance the chassis when installing SPA and SIP? The Cisco ASR 1006 has 12-SPA, 6-RU, hardware redundant chassis with two ESP slots, two RP slots and three SIP slots.