Based on understanding, the CPU on 1001-X is more powerful and can handle more concurrent session establishments. It also has 2 10G interfaces which can come in handy. However, the queue-count is considerably lower in the 1001s which makes QoS difficult, depending on your policies. I would recommend a 1002-X if you are looking at ASR 1K.

 

Have a look at this document here for more details:

 

http://www.cisco.com/c/en/us/products/collateral/routers/asr-1000-series-aggregation-services-routers/datasheet-c78-731640.html