Which is better option SAP XI vs webservices to interact with Biztalk

HI everyone,

We have biztalk as integration tool. I would like to know is it feasible to implement Biztalk with SAP XI to communicate with our SAP ECC server. I heard it will be over weight to use both Biztalk and SAP XI coz each will have its own message box and leads to more process time. So we are planning to use Biztalk with SAP webservices. So please let me know which is better as cost wise and work wise biztalk with SAP XI or Biztalk with SAP Web services.

SAP Guys please take some time to provide some useful information. it will be much appreciated.

cheers

ram