RTX 3090 vs RTX 4090: which used card for local AI?

Same VRAM, same models

Capacity decides what you can run, and here the cards are identical: 24GB fits 30B-class models at Q4 (Qwen3-coder 30B, DeepSeek-R1 32B distill) with room for context. Neither reaches 70B dense models alone.

For single-stream chat, LLM token generation is mostly memory-bandwidth-bound — the 3090 moves about 936 GB/s to the 4090’s roughly 1,008 GB/s, so generation speed is closer than the raw compute gap suggests. Where the 4090 pulls far ahead is prompt processing, batching, and anything compute-heavy.

Where the 4090 earns its premium

Image and video generation is the clearest case: SDXL and Flux run roughly twice as fast on Ada. Fine-tuning and LoRA training see similar gains, and the 4090’s FP8 support unlocks faster paths in modern inference engines that Ampere never gets.

The 4090 is also two years newer, which usually means less wear, more remaining board life, and stronger resale when you upgrade.

The dual-3090 wildcard

Two used 3090s often cost about the same as one used 4090 and give you 48GB plus NVLink — enough for 70B models at Q4 with llama.cpp, exllama, or vLLM tensor parallel. That is a capability tier a single 4090 cannot touch.

The tradeoffs are practical: roughly 700W of GPU load, serious case airflow and PSU headroom, and multi-GPU setup friction. If you are comfortable building, it is the best sub-workstation path to 70B.

Condition and shopping notes

3090s are 2020–2021 cards and many mined: ask about thermal pad replacements, listen for fan bearing noise in videos, and assume no warranty. Price accordingly — our fair value already assumes a clean, working unit.

4090s are newer but check the 12VHPWR connector closely — ask for a photo of the plug end. Melted connectors are the model’s known failure story, and a discolored plug is a walk-away sign.

FAQ

Is the RTX 3090 still worth it for local AI?

Yes, at or below fair value. 24GB runs 30B-class models at Q4 comfortably, and no newer card matches its used dollars-per-GB of VRAM. Check the live median on this page against our fair value before buying.

Is the RTX 4090 twice as fast as the 3090 for LLMs?

Not for single-stream chat — token generation is memory-bandwidth-bound, so expect roughly 1.2–1.5×. For prompt processing, batched inference, fine-tuning, and image generation the gap approaches 2×.

Should I buy two 3090s or one 4090?

Two 3090s (48GB, NVLink) win if your goal is running 70B-class models locally. One 4090 wins on simplicity, power draw, image generation, and speed on anything that fits in 24GB.