I would say a KFX400 or LTZ400 same machine. They are not as fast as the 450's but they have reverse, super easy to steer, a low 1st gear(easier starting / taking off) and have a big comfy seat. If your truly buying an atv for your wife and not a second one for you then you should deffinatly look at these two machines.
Big bore atv's are also nice because they can leave them in 2nd or 3rd all day long and not have to do to much shifting.