AWS c5n.4xlargevsAWS c5n.9xlarge

c5n.4xlarge pairs 16 vCPUs with 42GB of RAM at $0.8640/hr On-Demand (about $622/mo at 24×7). c5n.9xlarge pairs 36 vCPUs with 96GB at $1.9440/hr (~$1400/mo). c5n.4xlarge is 125% cheaper per hour than c5n.9xlarge ($1.0800/hr gap).

Because both instances are in the **c5n family**, the only thing that changes between them is sizing — same silicon, same architecture (Intel Xeon (x86_64)), same burstable/sustained behavior. The choice is purely about how much capacity you actually need: c5n.4xlarge gives you 16 vCPUs and 42GB of RAM, c5n.9xlarge gives you 36 vCPUs and 96GB. AWS scales pricing close to linearly within a family, so picking the right size is mostly about right-sizing your workload, not getting a better deal per vCPU.

On raw price-per-performance, the two are c5n.4xlarge delivers ~122% more single-thread Sysbench score per dollar (1344 vs 605 points per $1/hr). That's the cleanest signal we have for "which one runs your workload faster per dollar," but it only matters if your workload is single-thread-bound; for parallel workloads the multi-core scores (14271 vs 32260) are what to weigh. Spot pricing flips many of these comparisons — when c5n.4xlarge drops to $0.3711/hr and c5n.9xlarge drops to $0.6398/hr, the cheap-per-hour winner can swing meaningfully.

In practice, pick c5n.4xlarge when your workload is closer to compute-optimized (CPU-bound work — batch processing, web servers under sustained load, game servers). Pick c5n.9xlarge when it's closer to compute-optimized (CPU-bound work — batch processing, web servers under sustained load, game servers). When neither side is obviously right, the cheaper hourly rate usually wins for fault-tolerant batch workloads, while the higher single-core score usually wins for latency-sensitive web traffic. The regional pricing tables linked from each instance page below show where each is currently cheapest — sometimes a >20% regional gap flips the comparison entirely.