DGX Spark vs Mac Studio M3 Ultra for Local LLMs: Prefill vs Decode

Short answer

Choose the DGX Spark if your priority is:

• coding agents that feed long project prompts
• fast prefill and time-to-first-token
• CUDA, TensorRT-LLM, and FP4 model support
• clustering two units for 200B+ models

Choose the Mac Studio M3 Ultra if your priority is:

• fast decode on long generations
• up to 512GB of unified memory for the biggest MoE models
• macOS as your daily machine
• quiet, low-fuss operation with MLX and llama.cpp

Specs that matter

Apple lists the M3 Ultra at 819 GB/s with configurations up to 512GB. NVIDIA lists the Spark at 273 GB/s with about 1 petaFLOP of FP4 compute. One line explains every benchmark below: the Mac has roughly 3x the bandwidth, and the Spark has several times the compute.

The measured split: prefill vs decode

EXO Labs benchmarked both machines directly on Llama-3.1 8B at 8K context. The Spark’s prefill ran about 3.8x faster than the M3 Ultra. The M3 Ultra’s decode ran about 3.4x faster than the Spark.

Prefill is compute-bound, so the Spark’s FP4 muscle wins. Decode is bandwidth-bound, so the Mac’s 819 GB/s wins. Neither machine is “faster.” They are fast at opposite halves of inference.

Map that to your workload:

• Coding agents and RAG: prompts are 16K–64K tokens, answers are short. Prefill dominates. Spark.
• Chat and writing: prompts are short, answers are 500+ tokens. Decode dominates. Mac.

Giant MoE models: closer than the spec sheet suggests

At the 397B-class Qwen tier, alooftwaffle’s testing measured the M3 Ultra at 26.7–29.2 tok/s decode versus about 27 tok/s for two clustered Sparks. That is a tie on decode. But the Sparks ran prefill 2.3x faster: 730 versus 317 tok/s at 4K context.

For batched serving the Spark side pulls further ahead. NVIDIA’s developer forum shows dual Sparks pushing about 235 tok/s of Qwen3 235B NVFP4 throughput on TensorRT-LLM. The Mac’s advantage is capacity: a 512GB M3 Ultra holds models no single or dual Spark configuration can. Our 128GB RAM model guide covers what fits at each tier.

The price changed: $5,299 as of August 2026

The M3 Ultra Mac Studio launched at $3,999 in March 2025. Apple raised it around June 2026; MacRumors and Macworld report the base now starts at $5,299 as of August 2026. That flips the old framing where the Mac was the cheaper box. The Spark, at $4,699 after its own February 2026 hike, is now the less expensive machine.

One caveat before you order the Mac: 9to5Mac reports a Mac Studio refresh is rumored for late 2026. If you can wait a quarter, wait.

The hybrid option

You do not always have to choose. EXO Labs ran the two together: Spark handles prefill, M3 Ultra handles decode, KV cache streams between them. The combination measured about 2.8x faster than the M3 Ultra alone. It is an enthusiast path, but it proves the point: these machines have complementary, not competing, strengths.

Decision table

Final recommendation

For agent-heavy OpenClaw work with long prompts: DGX Spark. Prefill speed is what you feel every request, and it is now the cheaper machine as of August 2026.

For long generations, giant MoE models, or a Mac-first life: M3 Ultra, ideally after the rumored late-2026 refresh resolves.

Next steps

• Run the Local LLM Fit and Speed Estimator
• DGX Spark vs Strix Halo for local LLMs
• Best local LLMs for 128GB RAM

Sources

• https://blog.exolabs.net/nvidia-dgx-spark
• https://www.apple.com/mac-studio/specs/
• https://www.nvidia.com/en-us/products/workstations/dgx-spark/
• https://alooftwaffle.substack.com/
• https://www.macrumors.com/