The i5 is about 10% faster in multi core applications (like some games) and has the general advantage of beeing a real quad core. For everyday work like excel you will most likely not see any difference between these two.
In terms of upgrade path the i3 would be the better choise, as the architecture is newer and you will have the option of more mainboards that support them (because 3rd Gen Mainboards are going rare), as well as the option to go to an i5 or i7 of the 6-Gen Intel Lineup later.
So overall it's your choise but I would probably go for the newer tech.