I mean this in the friendliest possible manner :
If you have the kind of money to consider these options, limiting yourself to two cars (and ESPECIALLY one dealer), is shooting yourself in both feet. Also letting someone else decide or limit you is just a complete lack of self respect and balance in your relationship. If your wife makes her own money, you probably wouldn't tell her she couldn't buy a $2,000 coach bag right? Why should she be such a downer? Whatever, not my business really, but that's not a relationship that I could live with.
On to the cars :
The V6 CTS is a nice car. It's not a step up from an IS350 though. If you don't care about extreme performance, it's decent enough.
The IS-F is kind of silly, mainly because it's just not a really good match to the chief competitors. It looks goofy, and isn't well respected vs. the M3, C63, RS, etc. It's also, crucially, not a huge upgrade from an IS350 unless you were going to track it seriously. An IS350 will put you back in your seat, and give you ~5 second 0-60 runs, handle fairly well, etc. The IS-F ups the ante a bit in most performance aspects, but not hugely, and most of the gains would only truly show up on a track. BMW faces a similar dilemma with the 335i being extremely good. The M3 is better, but only real performance enthusiasts would really appreciate the difference, and it's a pretty solid gap in price. And the M3 actually looks fantastic, instead of a plasticy faux ricer look that the IS-F has. In fact, if you were to *give* me a free IS-F or free IS350, and I couldn't sell it, I'd actually take the IS350.
This leads me to my conclusion. The IS350 is a great car. You made a good choice already, keep it. If you truly want something higher performance, then don't mess with the IS-F. Have you driven something with low-end torque? If no, then check out a Vette GS or C63, it's a pretty astonishing difference, it's a lot more satisfying feeling than say an M3 to feel immense torque at low RPM, even if the end result in raw performance numbers ends up fairly similar. GT-R is not much more than the IS-F, and it completely obliterates it at every level.
So yeah, keep IS350, or consider :
S5
M3
Vette GS
Cayman S
911
C63
GT-R
CTS-V (duh, lol)
etc...
/my 2 cents, best wishes to you, no offense intended, just raw honesty for ya.