Starting with the CPU section where both, the Dimensity 800U and Dimensity 720, have octa-core CPU based on 64-bit architecture. Speaking of the Dimensity 800U, it has Two ARM Cortex-A76 Big Cores running at up to 2.40GHz and 6 ARM Cortex-A55 Small Cores operating at up to 2.00GHz clock speed while the Dimensity 720 contains 2 ARM Cortex-A76 Performance Cores working at up to 2GHz and 6 ARM Cortex-A55 Efficiency Cores that can be operated at up to 2GHz clock speed.
Here both have the same CPU architecture but the Dimensity 800U has a better clock speed in comparison to Dimensity 720. So in terms of CPU processing, the Dimensity 800U is better than Dimensity 720.
Moving on to the GPU section where the Dimensity 800U and 720, both have an ARM Mali-G57 MC3 GPU. Here both have the same GPU and the clock speed of both is unknown but we assume Dimensity 800U to have a better clock speed compared to Dimensity 720.
In this section, both SoCs are the same because both are fabricated on the 7nm process. So here both are power-efficient.