Hi Keyur,
In terms of config, there are no major diferences between one and the other one because either one allows you to run the same code. In other words you could have an ASA 5540 or an ASA 5585 running OS ASA 8.2 for example and their configuration would be the same. You could copy the configuration from one device to another one, you would just need to make sure that you have the same or higher number of interfaces available.
The major differences between the different ASA models are hardware capabilities (number and type of ports, modules available, etc) and performance and license capabilities (throughput, number of concurrent connections, number of VPN/SSL connections,etc) but in terms of configuration per-se the configuration would pretty much the same, limited to what the hardware allows you to do (for example the small ASAs dont allow you have several VLANs or ASA Contexts).
Here is a model comparison for ASA 5500 series:
http://www.cisco.com/en/US/products/ps6120/prod_models_comparison.html#~high-end
I hope this helps.
Raga
