Swings and roundabouts on that one, two folders one for xls the other for txt or two loops one for xls the other for txt will provide exactly the same methodoligy.

But then you have to look at if there is a process between them, say for example the xls is header information and the txt is the data and they need to be imported xls, txt, txt, txt, txt etc if so then two loops wont cut it as they will always insert in parallel or serial depending how you setup the precendence constraints etc.