Well a few questions,

1)Are you going to be transitioning entire applications at one time or will applications be running in parallel on both platforms?

2)How much data are you talking about transferring, either at one time or on a daily basis?

3)If you are planning to transfer data daily what options do you have for trapping for new/updated records on the legacy platform and are there any restrictions based on the data like identity columns or enterprise id's generated by the application that can't be migrated?

4)Do your applications have any built in data migration tools?

Linked servers aren't a terrible option if you're talking small data sets(10000's of records) and they're likely much easier to set up and use than an ETL tool of some kind.