Which one should I buy between the sigelei 100 watt

or they sigelei 150 watt?? I really don't know the difference between the two.
The difference between these two mods is of course the power output of 100 vs 150 watts and also that they handle 0.15 ohms minimum vs 0.10. Both have very good reputations as dependable devices. They are both big and heavy, but the 100 watt is a bit smaller and a bit lighter. Both do not boost voltage so will be voltage limited to the sum of your battery voltage, with around 7.5V as the maximum and as battery voltage drops that will go down.
I have a Sigelei 150. It's a great device and the 0.10 ohm minimum resistance lets me use my temperature control atomizers on it with low resistance nickel wire builds if I chose to do that. When I work with Kanthal I generally build at between 0.3-0.7 ohms so that's not an issue.
I have experimented with high power builds at 100 watts plus and I don't like them. The vast majority of vapers stay well below 100 watts. I typically vape between 25-55 watts.
IMO, if you want to experiment with really high power vaping go with the 150. If that isn't your cup of tea, go with the 100.