I have both. Both are good. They use the same board so performance differences aren't an issue. It is all about build quality, ergonomics and esthetics. Build quality for both are good and in line with their price point. The Sigelei is slightly shorter and not as wide as the ipv but the ipv has the rounded back and some people might prefer that. Both seem pretty sturdily built. The display on the sigelei appears brighter and easier to read in direct sunlight but the ipv mini isn't that bad. The IPV appears to use a reflective covering over the display which is probably why it isn't as bright as the sigelei but is certainly not as bad as the early ipv4s which are completely unreadable during the day.
I prefer the sigelei because it feels lighter and the size makes it a little more convenient to hold and lug around. I also like the brighter display but these are subtle differences and you won't go wrong with either.