Both can be done at the same time and fairly easily in my opinion. It all depends on how disciplined you are. I recently went from 280 to 229 for a deadlift meet. I would have smoked a PR but I tore my hamstring on my 2nd attempt. Eat clean, eat frequenlty, do cardio, take creatine, eat a shitload of good fats, and drink a shitload of water.
Thats about it. I am already back up to 270, still eating clean but am doing less cardio and added in 2 cheat meals a week. I got my bodyfat tested in lab conditions this week and it had me at 19%. When I was at 280 I was probably around 25%-30%. I am going to add in some cardio again to get back down to 260 for these new stupid IPF weight classes.
Any weight lost will cause your bench to go down but, not a lot as long as your program doesnt suck and you dont freak out and add a shitload of volume. Personally, all of my testing exercises went down about 3% and leveled off there when I was at my lightest. Might not sound like much but thats the difference between 970lbs and 1000lbs.
Anyway, I am not trying to impress anyone, I am just saying you can lose fat and get stronger in the powerlifts. Just when you get to the point where your total starts to suffer, you really have to figure out which is more important to you.