Suppose I have 3 unspent outputs:

tx: 3 BTC
tx: 2 BTC
tx: 1 BTC

Now I want to spend 2.5 BTC.
Which is preferred as input(s) and why?

A single:
3 BTC (Leaving my an output change of 0.5)?

Or:
1 BTC + 2BTC? (Leaving my an output change of 0.5)

I hope the question is clear.