Sell my 3090 for a 9070 XT or just keep it?

If those prices actually work out, I'd make the swap for a gaming machine. You could get more gaming performance and some cash back, provided the games you play don't need the extra VRAM.

Check completed sales rather than asking prices, then subtract selling fees and shipping from the 3090's sale price. The $300 difference could shrink once everything is accounted for.

I'd hold if you're at all interested in running local language models. The 3090's 24GB can be useful there, even if it isn't fast enough for every model or every person's expectations. The 9070 XT has 16GB, so this isn't a free upgrade in every respect.

Look at the games and features you actually use before selling. Moving to Radeon means giving up NVIDIA features such as DLSS, and upscaling support can differ between games. Compare ray tracing results too if you like playing with it enabled.

When switching cards, clean out the old graphics drivers and install the new ones. A driver cleanup tool is an option if you run into problems.

Don't count the sale money until you have a buyer. Selling a used GPU can involve fees, shipping, returns, or disputes. Compare your options and pick a way to sell that you're comfortable with, especially if you need the proceeds to fund the replacement.

Another option is a 7900 XTX if you want to keep 24GB while moving to Radeon. Compare its current price and gaming benchmarks against the 9070 XT, though. If the extra memory has no use for you, the newer card may make more sense.