I have both packs, and if I were to pick one I'd go with the Porter simply because it's a more efficient system. I've used the Epic for three solid summers of packrafting adventures, and the weaknesses are twofold: 1) the load hangs low, and 2) it's difficult to pack fast & efficiently, especially when carrying heavier loads. I think the Epic still holds a place among "raft packs", especially day boating with a 35-liter bag...but the fact is that Alpacka, since they came out with the zipper boat, has effectively eliminated one of the problems packrafters face: the need for a waterproof pack.
Now, breaking it down to basic economics, let's assume you have limited funds and the choice of either getting a new pack designed for packrafting, or the zipper boat retrofit for your Alpacka. The cargo fly retrofit is currently $275, and it includes two dry bags for gear storage inside the tubes. Unless you use a huge expedition pack, whatever you have should fit empty inside the raft, totally protected unless you suffer the extreme misfortune of a torn tube. And with your gear stored at your sides along the stern, your center of gravity is lowered for the "fringe" benefit of increased performance in rougher waters.
Compare this to purchasing a new pack...and for the sake of argument we can even toss in the Exped Lightning 60 because it's in the same class as the Porter and Epic. The Porter is $345 for the 4400 model...the Epic remains $275...and the Lightning is $289. The only truly waterproof design is the Epic, and even then the suspension system is subjected to getting wet unless you have enough space to put the harness inside the dry bag. I've personally never had an issue with it being wet, but it's something to consider.
Some might be concerned about the reliability of the zipper boat, but in my mind that concern was laid to rest when guys started boofing 20-foot waterfalls in those boats without failure.
My point to all of this might seem obvious, but in case it wasn't....if you have an adequate pack that can comfortably carry all of your packrafting gear, but it wasn't specifically designed for "rafting the pack"...consider the zipper.