What would you use

What kind of car?

I have a 71 Challenger that needs a trunk pan. My thought was to get the one piece and cut out what I needed.

1 piece will probably require the removal of the tail panel to install.

68 road runner. Going to get a tail panel too

A couple of members here say they were able to slide a one piece in under the tail panel on an e-body. I've never seen it happen but it may be possible. Do a search it's definately been talked about but it depends on the car.

Then I would get a 1 piece, it is going to be "correct" looking. The only reason 2 piece are made is to install without removing the tail panel.

I just finished installing a two piece trunk in my 74 cuda. You can make it look fine if you take your time, I was concerned about the center seam, but most of it is covered anyway by the tire hold down and the trunk latch support.
Pro's of the two piece for me were; Ease of handling, I had this trunk in and out about 20 times before I welded it. RR trunk is much bigger than the cuda one, I know because they sent me that one by mistake the first time.
Price, I bought this for less than half what a 1 piece would cost.
Cons are, you have to weld the seam. Not sure with the RR but on mine I hade to weld the tank supports on the bottom. The drain holes and access holes to the shocks are not drilled out, easy fix. and the two piece panel does not extend to the tail panel on the cuda, so you have to make a piece to connect it. I would do a 2 piece again if I had to.

If you are replacing the tail panel then go with the one piece! I just installed a one piece in a 69 Charger and it fit like a glove.The one that I installed was from AMD.