I'm doing a facelift and Sculptra, which is better to do first?

Replace the volume loss AFTER. That way you get it put in the correct place, in the right amount, after the facelift puts the majority of tissues back where they came from. Sounds like you’ve got 2/3 of the plan figured out; facelift to put the tissues back where gravity dragged them down from, Sculptra or fat to replace the volume lost from aging, and what’s left is treatment of the effects of sun damage on the skin, ie, resurfacing of some type, be it laser, chemical peeling, radio frequency energy.
The garnish on that plan would be use of neuromodulators like Jeuveau, etc for any muscle wrinkles if required.

Yours is a good question. Your comment about faces sagging as well as shrinking is exactly correct. And there are multiple good options as far as timing. To answer your question specifically, photos would be required. A good analogy is you can drive from one one town to another by different routes, and still arrive safely.  But the best route depends on traffic, etc. 
There are some important distinctions between the modalities. I do a lot of Sculptra as well as fat grafting. Both options work far better and more economically than fillers (one cc at a time) for restoring lost facial volume.  With Sculptra, most of us National Trainers feel like each vial yields roughly 4-5 cc of collagen synthesis. With fat, we put between 40 and 60 cc.  Yield is estimated at 75-85%.  So, if you need just a little volume, Sculptra can be injected in an outpatient setting. It can be done before, during, or after the lift.  Your surgeon should give you his or her best estimate on how many vials, what areas would be treated, etc. Whereas high volume fat grafting requires sedation.  It is rare that we do a facelift that does not include fat grafting. That is the ideal time to avoid two recoveries and two OR fees.  
One caveat is that you are a bit on the young side for fat grafting.  As people age, menopause and orthopedic issues can sometimes result in weight gain later in life.  Fat which is grafted tends to behave as it would have if it remained in its donor site.  So I would tend to be a bit conservative in terms of volume of fat transferred and realistic about your future weight management.

My recommendation would be to have a facelift first, and then have the Sculptra treatments after everything has healed.  Both procedures can help to rejuvenate the face in different ways and they frequently can compliment each other.  For some people who do not have a lot of skin sagging, Sculptra by itself can provide enough benefit so that a facelift can be postponed until further aging has occurred.