Are you buying new or used? What sort of % spread will you be using this car versus the Z (50% of the time...)? Is it just you driving both?
I would find it hard to justify a new car for just 50% of the time, especially since it's being driven in the winter as a "beater" equivalent.
- Go used beater. Really, anything as long as it runs and does what you need.
- If you go new, I'd suggest a quality econocar around $20k that can still be sold in 10 years when you get her up to 150k+ miles as well. Toyota, Honda, whatever lasts a good deal on the inside.
If you can swing new and swing solid at $45k and lower, then get whatever I guess.
I would just then say don't get something sporty or anywhere close to the same category as the Z. Don't make it an agonizing choice which car to drive on a sunny day to work. If it's agonizing, you may as well sell the Z and get the year-round car, ya know?