Size "P" or 4x6 filter? Which one?

[wedding photographer]

I would like to buy a few natural density graduated filters. Lee or the Singh-Ray ones... I am not sure which size I should buy. I would like to use them with my Canon 17-40mm f/4 lens (filter size 77mm) or I can borrow my friends Nikon 14-24mm 2.8g lens.
Which filter size should I buy? The 4x6 is twice as much...
Thanks!

[wedding photographer]

I got the P first. Vignetting! Get the larger one....!

[Greg McCary]

If you want wide with no vignetting get the wide angle Cokin holder........
http://www.adorama.com/CKBPPW.html
Unless of course you want to stack filters. Make sure the filter you buy states it will fit a P holder.
But I think you have it. I shoot 67mm and 72mm and I use the P. I think this is the case
unless your lenes are huge.
Cokin also sells a three filter grad kit with everything but the ring and it cost not much
more than one filter would cost.

[wedding photographer]

If you want wide with no vignetting get the wide angle Cokin holder........
http://www.adorama.com/CKBPPW.html
Unless of course you want to stack filters. Make sure the filter you buy states it will fit a P holder.
But I think you have it. I shoot 67mm and 72mm and I use the P. I think this is the case
unless your lenes are huge.
Cokin also sells a three filter grad kit with everything but the ring and it cost not much
more than one filter would cost.

I want to shoot 17mm wide on full frame....not all the time but if I do I don't want to have vignetting....