If you had to pick one, I'd pick whey for post-workout (within 1.5 hour) supplementation. That said, it's possible to get big and lean with none of the above, it's just a bit more difficult to get the nutritional needs from real foods, although that is the preferred method.
Creatine has an important place as well, but I don't think it's indispensable. If you're trying to lose weight (read: bodyfat), creatine isn't exactly necessary, although it may help preserve lean mass.
Since you're trying to lose bodyfat, watch the total amount of calories you're taking in. If you're eating a lot real food and that whey supplement puts you at a 300 calorie surplus, then that's not conducive to bodyfat loss. Check out some of the posts discussing macronutrient ratios (protein/carbs/fats) for losing bodyfat while preserving lean mass.
Eat lean steak a few times per day and you'll hit both the protein and creatine targets! Due to your goals, I'd probably recommend the whey shake and a BCAA supplement after workouts. That should help prevent catabolism while you're running at a *slight* caloric deficit. Good luck!
EDIT: there was a similar thread yesterday. check out JoeBAR's response:
Protein intake w/ caloric intake reduction (newbie question)
Whey or Creatine? Neither? Both?
