.22 for majority usage as target shooting with some hunting/pesting. .25 for mostly hunting/pesting and a touch of target punching.
I was looking at the Hatsan Flash, and was going to buy it in .25. Then I saw somewhere that the .25 has a loopy trajectory, whereas the .22 is flatter. I don't know if this holds true for other rifles, but it caused me to change my mind to .22, and that's what I ended up buying. Most of my shooting is going to be small pests at short range, or basement target shooting.
The nice thing about the 22 is not only are pellets cheaper, but you can walk into any Walmart and pick up a tin or 8.
I have got both .22 and .25 and if I had to make a choice of only one to keep it would be the .25, it just makes me smile more when shooting it.