What is best for concatenating values in QString: arg(...) or +?

[stampede]

First, is there another method to do this?

Sure, you can use append() or push_back(), operator +=, or even sprintf, but I really doubt you will notice any change in performance with such micro-optimizations.

Second, which of them is the best for situations like the one mentioned above?

The most readable one (for me the "arg()" method, but it is a matter of personal preference).

what about speed and memory efficiency?

The thing you are doing in the background and stuff related to gui will have much greater impact on the performance than actual string composition.
Anyway, if you are unsure, then measure, do some performance tests.