Would it be preferable to gain more muscle (to that of a fitness model) before or after I get a breast lift?

Muscle gain in your pectoral area is unlikely to offset the fat loss in your breasts you are anticipating. I believe you are more likely to know what you  will end up with is for you to wait on your lift till you are at or near your final level of fitness. Good luck.

I would recommend completing your weight loss prior to surgery. Your plan for developing more muscle mass will not affect your surgery.  

Thanks for your question. You are correct in your thinking. Increasing fitness may increase your muscle mass, but it may also diminish the fatty volume of your breasts. If you have a lift first and loose this volume you will gain more skin laxity. In my opinion the increased muscle mass is unlikely to compensate for the fatty volume loss.The safest route would be to accomplish your diet and exercise goals before breast lift.I hope this helps you.

Chest muscle enhancement does not improve the shape of a womans breasts. Breast Shape and volume is determined by the amount of fat and glandular tissue within the breast. If you lose some breast fat, the volume will certainly decrease, However the shape would likely stay the stay in a fit, lifted breast.If you would like more information on breast lifts, please read my book " Cosmetic Breast Surgery - a complete guide from A to Double D".

Adding muscle volume will  not add significant volume to the breasts. But, if you are planning a significant transformation of your body, it is fine for you to wait before you proceed with surgery.

A change in muscle mass in itself should not affect your breasts. Your
breasts are made up of gland tissue and fat, and strengthening and
building muscles underneath will not change them.
If
you burn fat as you do weight training, however, it is certainly very
possible that you will lose volume in your breasts. If you're like most
women, you probably lose fat in that area before you lose it
elsewhere. The safest thing to do, then, would be to wait until your
training regimen is well underway before having surgery.
If
you are reluctant to wait, you can always have a breast lift
consultation and talk with your plastic surgeon about your case.
Together you can review your history and your plans and formulate a
strategy.

A breast lift procedure in itself is not affected by your muscle mass. However, depending of the composition of your breast tissue and the amount of fat tissue within, a body fat change especially weight loss of more than 10% of body wright will likely affect your breast size. You should undergo a dramatic weight-loss prior to a lift. 

If your breasts are more fat than glandular, then weight changes may impact the size and shape. Hard to say even with an exam.  Best of luck.

Breasts tissue is not muscular and I do not believe that muscle development should delay your desire to undergo a mastopexy.  Certainly as you build muscle, the appearance of your upper torso will be enhanced.  Just make sure to maintain your weight so as not to lose breast volume.

Your muscle has nothing to do with your breast. Breast lift techniques focus on tightening the skin envelope to fit the breast core. However, being active will positively affect your experience and outcome.