Reasons to get a Nismo vs. Base plus Sport:
1. No intention to mod or very light modding.
2. Prefer the looks of the Nismo.
3. Want the additional exclusivity of the Nismo.
I have one for reasons 2 & 3. If you plan on tracking your Z and doing a lot of upgrades you're probably better off getting a base + sport.
Just remember that in the end it's your decision.