Which Sub Box?

I've been contemplating a sub box for a long time and can't really decide, so I figured I would ask your advice. I'm considering a box that will go behid the 3rd row and will hold 2 12's. Both are sealed, one is down firing and one is forward firing. What do you think will sound best? If you have any other suggestions, let me know.

down firing
http://www.lightav.com/car/boxes/xscorpion/x8300.jpg

forward firing
http://www.lightav.com/car/boxes/xscorpion/x6395.jpg

I had a custom built box that fit securely behind the third row in my 04 Z71, it was downfiring and the thing would hit like no ones business. I'll try to get some pics of the box and post them up for you.

I think I saw in another thread that you had a 07 denali also, apologies if I'm wrong. Are you planning on putting subs in this vehicle also? If you have time, post pictures of your box. I'm not really needing that much bass. I really like the idea of one 10 in the side panel like OCKevin, but it would probably too much and money is an issue. I don't know how much it cost, but I had a shop quote me something over $600 for 1 jl10w3 in that location and they said it could possibly be more. They said they weren't sure and wouldn't really be able to tell till they started doing it. This pretty much made me give up on that idea. I own a pair of 12's that I haven't used for a while and have a okay amp which is broke, but I was quoted a price of 80 to 160 to repair it. I figured I would just get an okay box and this should actually give me a fairly cheap system.