Hi Isaac, I have both an eVent Packa and a MYOG version that's similar. I haven't owned the eVent version for long, and have only used it a few times, so not enough for a full assessment, but here's what I've found so far:
The eVent version is nice when you are wearing it all day and you have to seal it up. I've used it a bit in conditions where I had to completely zip it up and tighten the drawstring in the hem. It's really comfortable in this mode compared to silnylon. I bought it for this reason, for use in cold, windy, wet conditions where it would be worn pretty much all day. That and because it's eVent, which can't be purchased for MYOG projects.
The silnylon "mountain poncho" is much lighter, weighing about nine ounces in my version. The sil version will be much easier to deploy, and will stuff to a much smaller package when not in use. Using the 2-way zip and the pit-zips, it can be worn as a cape to mitigate the fabric's lack of breathability. It will be uncomfortable during warm heavy rains when you have to seal up the front and hood. I think, because you are less likely to need to seal it up for sustained periods during warm weather, and because it is more likely to be packed than worn, that the sil version is more appropriate for warm conditions.
my 2 cents