Depends om where you're going.
Winter time is great for game viewing, but the veld is really dry and brown. Summer the veld is lush and green, but obviously the game is harder to see. In winter the game tends to congrigate around water holes, but in summer it disperses, as there are little pans and water holes all over the place. Winter is May - September, but in the bushveld temps can still get to about 25-28 degrees celcius in the day, but drop below freezing at night. In the Summer, its warmer.
Still all depends on where you're going.