The first difference is the price. Gallardo is about $70K more than the R8.
Gallardo has also much better engine. It has a 5.0L - 520BHP - 375ft-lb V10 engine. Whereas the R8 has a 4.2L - 420BHP - 317ft-lb V8 engine.
R8 is much more stylish than the Gallardo, though. It is more of an attention grabber than the Gallardo.
But, Gallardo is more reliable than the R8.