There really is no difference in design between the Galaxy M31 and the M30s. Both phones have a plastic frame and a polycarbonate back. The vertically oval fingerprint scanner is also present on the M31.
M31 has a wider camera module to cram in an extra sensor. Besides that, Samsung has gone on a dark route in terms of color options on the Galaxy M31.
m31 have 6.4-inches FHD+ Super AMOLED panel with the same Infinity ‘U’ cutout for the front camera. This glass panel is protected by Corning Gorilla Glass 3 and has a peak brightness of 420nits. Since it’s an AMOLED display, its vibrant, colorful and has a great contrast ratio.
Battery
Samsung Galaxy M31 Specifications:
Display: 6.4-inches Super AMOLED panel
Resolution: FHD+ (2340 x 1080 pixels); 19.5:9 aspect ratio
Chipset: Exynos 9611; 10nm Mobile Platform
CPU: Octa-core (4×2.3 GHz Cortex-A73 & 4×1.7 GHz Cortex-A53)
Storage: 128GB UFS 2.1 (expandable using SD card)
Software & UI: Samsung’s One UI 2.0 on top of Android 10
Rear Camera: Quad-camera;

