Should I trade my RX 9070 XT for an RTX 5070 just for DLSS?

For 4K, the extra VRAM and raw performance matter a lot. The RTX 5070 is more of a 1440p card for demanding modern games, and you may need to rely heavily on upscaling at 4K. DLSS is usually better than FSR, but the gap has narrowed, and it does not compensate for the 9070 XT being substantially faster.

I would keep the 9070 XT. The RTX 5070 is a lower-performance tier, while the 9070 XT is closer to the RTX 5070 Ti in overall rasterized performance. Even with DLSS, the 5070 would generally be giving up too much performance and 4 GB of VRAM. If you specifically want NVIDIA features, it makes more sense to compare the 9070 XT with a 5070 Ti rather than downgrade to a 5070.

The main thing to check is the local price. If the 9070 XT cost far more than it normally should and you can return it for a much cheaper 5070, the savings could make the trade worthwhile. But if the prices are close, keep the 9070 XT; switching solely for DLSS would be trading down rather than upgrading.

I have used both AMD and NVIDIA cards, and I would still choose the 9070 XT here. FSR has improved considerably, and most people would prefer the stronger card with more VRAM over somewhat better upscaling and frame-generation features. Only consider the 5070 if it is dramatically cheaper or if you have persistent AMD driver problems with the specific games you play.

There is a reasonable NVIDIA argument if you strongly value ray tracing, path tracing, or DLSS and are willing to sacrifice performance. However, at similar pricing, the RTX 5070 is not a good replacement for a 9070 XT. You would likely notice the downgrade in native rendering performance, especially at 4K.