Website integration - Querying information in real time

I'm building a website that integrates into an accounts system (something like xero.com), I need to present the user with a list of invoices (from the accounts system).

Is it in general best practice to import the invoices into my own website (into a table) and then display it or just query the accounts system each time, bring the data back and display it (using paging, etc).

I could sync the data into a table every time the user logs in to my site, etc.

I plan to one day offer this as add on in the store, so the number of invoices could vary greatly.