It's a dilemma as old as time itself: Do you want an authentic, vintage part that may have quirks, or do you want a new, knock-off part that starts when you turn the key? Take into account your sense of authenticity, the specific part and the quality of the knock-off.
First, ask yourself what your goals are in restoring this car. If you cheat with a knock-off fender, will it make you value the car less when all is said and done? Or is looking classic good enough? If it's the second, new, imitation parts may be the best, cheapest and easiest way to go. (If it's the first, keep looking!)
Also, consider the difference between visual and functional parts. You can usually restore visual vintage to good condition if you find the right model/make/year it's guaranteed to fit, and it's not like a reclaimed fender is going to give you driving problems.
Finally, if you buy new pieces for classic cars, make sure you're not buying junk. Sometimes classic is better. If you can't tell the difference between quality classic parts and junky new parts, spend some time around friends' projects, swap meets, starter projects and rebuild shops before diving into your dream project.