I'm not sure my wife would appreciate the cylinder sub as much as I would!!!
If you are really not sure how she would feel about a cylinder, you can talk to her about it. The cylinder subwoofers tend to "disappear" in a corner of the room, particularly when the lights are low. They take up less floor space than a comparable box subwoofer, and are more lightweight than a comparable box subwoofer. You can also tell her how you are saving money, as the cylinders tend to be cheaper (both in official price and in shipping costs) than comparable boxes. And tell her you will take her out to dinner with the savings, or buy her something nice with that money; perhaps flowers and good quality chocolate.
If you can afford it, this would be what I would buy:
http://www.svsound.com/products-sub-cyl-pcultra_new.cfm
As rnatalli said, "When in doubt, buy bigger and better". Or just better.
And with this subwoofer, you can probably be safe in promising your wife that you will never upgrade or replace it, as long as it lasts. That translates to future savings, which she may appreciate as well.
In my case, I own two of the old SVS CS-Ultras (with the "little" 12" woofers), and I do not plan on ever upgrading. After all, with flat bass response down to about 15 Hz, and the ability to play room shaking loud, why would I upgrade? My wife is happy that I do not want to spend any more money on subwoofers for my home theater, and probably yours would be, too.