I had to make this choice yesterday myself. I decided to go with the new Sig 150TC, I own the old 150W, but actually quite like the angular lines, the material seems durable, the YIHI chip (I know the old one was YIHI too, but an updated one) made me choose the Sig150TC.
I didn't do much research on the IPV 4 or 4S (Same price here in the UK, one a hundred watts the plus is 120) But as I own quite a few Sigelei devices and know they are solid, I went for it.
I got that as the Sig 75TC was not really as impressive as I hoped for, and I do prefer the IPV d2 out of them both for TC. For out and about I use the sigelei 75W in power mode, or the D2 with the workaround at 20-25 watts with a tank, perfect take out vapes, but I also got a super cheap mod called the smowell that I have now been taking out, as if it gets damaged I will not be bothered.
But, if I want TC out and about, I will use the IPV D2, and the 150TC for when I know I need long battery life.
I personally would go for the Sig, due to P4U's past mishaps, plus the D2 needing a workaround for low wattage firing, I went for the Sig as I doubt there will be any bugs like that in the Sigelei. Not that its a major issue, and the workaround is a piece of cake, but I just would like the power mode to work fully, and not have to use TC mode with kanthal, Overriding the TC and adjusting joules instead of watts. But, remember the chinese did mess up, and they did not convert joules, they just renamed the watt to joules, so 1 joule=1 watt so for 20 watts, I select 20 joules.