Hi Nikhil,
I would use BDocs to integrate CRM and ECC, I would use IDocs to integrate CRM with another non-SAP system, a good thing to considere is: Which entity will be the middleware integrator?, if for example is CRM, then BDocs, if is an external middleware tool, like PI, MQSeries, etc. I would use IDocs.
You can add to all of this the XIF addapter, which you can use to take the advantage to all SAP CRM the middleware capabilities( monitor, triggers to push change of data, etc.) to map the Bdoc to an external comprensive message type like IDoc, xml or SOA.
If you want to replicate Z tables, you can do it via BDoc (if you search in scn there are some blogs which explain that, but I never did), via IDoc, or even via RFC/Webservice.
In my opinion, if you have enought time the best way should be use BDoc in order to take advantage of the SAP CRM Middleware technology.
Cheers!
Luis