Dude why are you looking at an IS250? Those things are slow as balls.
The IS350 on the other hand is a different story. Stock acceleration wise it's actually faster than the G37 in the 0 to 100 range (thanks predominantly to its aggressive 4.08 final drive ratio which lends itself to a sturdy 0 to 60 in 4.9 seconds as measured by Road and Track or 0 to 60 in 5.1 as measured by Car and Driver).
My younger brother has an IS350 so I'm well aware of its performance characteristics. Out of all the entry level luxury sedans, it's 2nd in its class in terms of stock acceleration (behind our beloved 335i's).
Three major cons of the IS350:
1) It drives like a total boat bone stock. You'll have to equip it with F sport sway bars to get it to handle reasonably well
2) There are NO
(I repeat, NO) performance upgrades for the IS350. What you see is what you get. The engine is N/A so apart from the intake/exhaust/headers for ~20-25 whp that you can add to most N/A cars, there's nothing else.
3) The transmission is PURELY automatic. The paddles on the steering wheel are not
shifters, so if you wanted a "manu-matic" that allows you to control the gear you're in, you'll have to look elsewhere.
I'd get a slightly used stock IS350 before touching an "F-sport IS250." The "F-sport" part means nothing. The IS250 is not fun to drive at all.
Example, IS350 (306 HP) vs G37 IPL (348 HP) from 0 to 80 MPH-ish:
The IS350's low end is pretty solid.
I agree with Nack54 that the G37's low end feels sluggish which could be a problem because the low end is what you deal with everyday on a daily basis. The areas where the G37 shines are more top end once you get > 100 MPH, but it's totally useless for daily driving.
A 6MT G35 is generally considered to be faster in a straight line than even a G37, surprisingly.
I've never looked at them but what about a Hyundai Genesis 3.8? Don't they have like 340 or 350 HP now? And I'm sure you'll get a really great deal on them because the "Hyundai" brand name is such a turn off. If you're looking for value, the Hyundai Genesis might be one way to go. But again, I've never driven one or even looked at the prices. I know they're going to be way cheaper than any BMW/Lexus/Mercedes/Audi.
Another car you might want to look at is a new Mustang v6 (~300 HP for around $25k base MSRP). Great value.