I think either way yo go you will be satisfied really. They are pretty similar construction. The sig has magnets and fires 10-150w The p4y is 7-165w with the update. They are sister companies the Sig is marketed like the Honda where the p4y is the Acura. If you like magnets and don't want to modify get the sig. If you like a bigger fire button get the sig. If you like a lower price get the sig. If you like a slightly smaller frame get the p4y. If you like the bb style buttons get the p4y. If you like 165w get the p4y. If you like a more detailed volt and ohm reading get the p4y. lol
I think they removed it at the last minute but did not want to have to change the enclosure.
I'm an iPV3 owner, too, and this post just about covers it. iPV3's can be found for about the same as Sig's, too. The only thing I envy is a magnetic battery door, but it's no big deal. I have a million little blue Phillips screwdrivers, and with two 18650's I'm yet to have to change batteries in a pinch, and need a screwdriver.
Another thing to consider is the memory feature on the iPV3. It erases after swapping batteries, but there are five power setting you can dial in to memory, and it takes a minute or so. It's convenient to be able to press one button one time to go from as low as 7 watts, to 165, though that's an extreme example. Usually mine has 40-45-50-55-60, which is an easy way to tailor in for builds and juices that differ. After having this feature, I know I'd really miss it if I were without it.
I don't think you can go wrong either way. The iPV4 is coming soon, it's 100 watts, and I could see the 3's dropping further in price. Might be worth the wait.