Mac Studio M5 Max: 64GB vs 128GB — Same 614 GB/s Speed, 4× Context Gap for 70B LLMs

Specs: Two Bandwidth Tiers

M5 Max memory bandwidth splits into two tiers: 32-core GPU version at 460 GB/s (max 48GB) and 40-core GPU version at 614 GB/s (64GB/128GB). For local AI, the 40-core + 64GB configuration is the minimum viable starting point; the 48GB base model loses both bandwidth (~25% slower) and capacity.

Pricing Overview

Estimated prices (based on Apple's historical upgrade increments): 36GB/512GB (32-core) ¥19,999; 48GB/512GB (40-core) ~¥21,499; 64GB/1TB (40-core) ~¥23,999; 128GB/1TB (40-core) ~¥27,999. The 64GB→128GB upgrade costs ~¥4,000 and buys only larger model support and longer context, not higher speed.

Capacity & Maximum Context: The Real Divide

Using Q4_K_M quantization, GQA architecture, FP16 KV cache, and ~4GB system overhead:

27B models : Both 64GB and 128GB handle them comfortably; context capped by software limit (~128K), no difference.

70B Q4 (~42GB weights) : 64GB leaves ~18GB for KV cache → max context ~30K tokens. 128GB leaves ~82GB → max context 100K–200K tokens. Same model, 4× context difference.

120B Q4 (~72GB weights) : 64GB cannot fit; 128GB leaves ~52GB → max context ~64K tokens. 128GB is the exclusive entry ticket for 120B-class models.

In short: 64GB runs 70B but cramped; 128GB runs 70B comfortably and unlocks 120B.

Measured Three Speeds: Prefill & Decode (Identical Across 64GB/128GB)

Because both configs share 614 GB/s bandwidth, prefill and decode speeds are identical. Estimates based on effective bandwidth ≈0.66×614≈405 GB/s, anchored to M5 Ultra 27B real-world 51 tok/s at same utilization, plus community ranges. Single-stream, Q4 quantization:

7B/14B : Decode 90/45 tok/s; prefill 120–180/60–90 tok/s (small models benefit from cache residency).

27B : Decode ~26 tok/s; prefill 35–55 tok/s — smooth for daily coding and long-document QA.

32B : Decode ~21 tok/s; prefill 30–45 tok/s.

70B Q4 : Decode ~10 tok/s; prefill 12–18 tok/s. 70B on M5 Max is bandwidth-bound ; prefill barely exceeds decode. A 4K-token prompt takes ten-plus seconds just to ingest.

120B Q4 : Decode ~6 tok/s; prefill 7–10 tok/s; only on 128GB.

Correction: Previous article merged M5 Max and M5 Ultra 70B speeds (20–25 tok/s). This analysis separates by chip: 70B Q4 on 614 GB/s M5 Max ≈10 tok/s; 20+ tok/s belongs to 1.2 TB/s M5 Ultra.

Why 64GB Feels Cramped on 70B

Two real-world scenarios illustrate the ~30K context ceiling:

Codebase QA/RAG : Indexing a medium repo (tens of thousands of lines) can consume 20K+ tokens. On 64GB, 70B context caps at 30K — only half the manual fits. 128GB ingests the whole library, drastically improving answer quality.

Long-document summarization / contract comparison : A 100-page PDF easily exceeds 30K tokens. 64GB forces truncation or context reduction; 128GB swallows it in one pass.

Speed identical, capacity doubled, context 4× — that is the 128GB value proposition.

Decision Guide: Who Chooses Which

Only 7B/14B/27B chat & coding : 64GB sufficient; 128GB waste. Must buy 40-core (614 GB/s), not 48GB base (460 GB/s).

Primary 70B, short QA/single-turn coding only : 64GB works (~10 tok/s), budget-friendly; accept ~30K context compromise.

70B for RAG/long-docs/whole-repo QA : Must have 128GB . Same 70B, context jumps from 30K to 100K+ — different experience tier.

Want 100B+ MoE or 120B dense : Only 128GB ; 64GB cannot load.

Not recommended : ① 48GB base (460 GB/s) as AI machine — loses on both bandwidth and capacity; ② Comparing Apple Silicon to same-price CUDA cards on single-model throughput — not its strength; ③ Assuming 128GB runs faster than 64GB — same bandwidth, same speed.

Bottom Line

M5 Max 64GB vs 128GB: the comparison is about boundaries, not speed. Both at 614 GB/s deliver identical per-model throughput. The extra ¥4,000 for 128GB purchases the capacity ticket for "70B with full long context + ability to run 120B." Decide based on the model size and context length you need to keep resident: if you want 70B with long context, 128GB is worth it; if 27B chat is your ceiling, 64GB is plenty.

Note: M5 Max memory bandwidth has two tiers: 460 GB/s (32-core GPU, max 48GB) and 614 GB/s (40-core GPU, max 128GB). 64GB/128GB both require 40-core GPU. Speeds are bandwidth-model estimates (effective bandwidth ≈0.66×peak, anchored to M5 Ultra 256GB measured 27B-4bit 51 tok/s at same utilization) plus community ranges, not per-unit measurements. Prices are estimates based on launch pricing and historical upgrade increments; verify on Apple's configurator. Confirm chip, memory, and bandwidth tier before purchase.