Hi Goatman
I recently got myself a PDF for kiting as I'm not much of a swimmer and also not getting any younger.
I went with the dawn patrol by Ripcurl as it was only a bit more bulky than an impact vest and actually fitted better. I think the main thing to think on is how much flotation do you want as an impact vest traditionally only gives you approx 10kg i.e it takes 10kg off your weight to make it a bit easier to float.
The coast guard approved vests have flotation ratings 1 being the best with a neck support so if you were knocked out you would rise with your head out off the water so to speak. 2 & 3 are the same but 3 doesn't have the approved colours i.e not bright/floro etc.
Try a few on is my recommendation and find one that isn't restrictive on your arms. I found the jet pilot ones a bit too bulky and the wake ones allot better but like a wetsuit it comes down to your bodies fit.
http://www.hyper.co.nz/ride/wake/detail/?id=1785