With the V3, you get the most modern and innovative operating system at this price point, and a gun made in the USA. The downside to the V3 lies in overall weight (compared to the SX4), generic aesthetics, and less lively feel.
The SX4 is merely an updated SX3, using the same old gas system as the Browning Gold/Silver, and the Winchester SX2 and SX3. The updates are all ergonomic. Collectively, these updates make for a slimmer, lighter and livelier gun than the predecessor.
I personally chose the SX4. My local shop had a couple in stock last week, and they just felt great. Compared to the SX3, the A300 and the V3; the SX4 is slimmer, the oversized controls and trigger guard are easier for me to use, and the gun just seems to have more inherent quality and attention to detail than anything else in the price range.
I live out west, hunt upland birds exclusively, and tend to choose light guns that are easier to carry all day and are quick to mount and shoot. If I were a waterfowl hunter, I may have chosen the V3 or Versamax.