If your wife is going all the time you will be better off with a tour box for her comfort.
+1 on Mort's suggestion. My wife did not ever ride and was not entirely comfortable leaning back on the backrest. A rider that does not feel comfortable and is tense will do all kinds of strange things to affect your control of the bike. Got a trunk with a semi wrap around backrest and it made all the difference in the world. Now I barely notice she is back there. Besides if your wife goes with you much, soon you will find yourself hauling around a lot more crap than you can cram into the saddlebags to keep her happy.
Plus if you live a ways from the city, you might find yourself running on the bike instead of the cage for small shopping trips if you have room to haul a little bit of stuff home. . . .