How does one choose between "fewer parameters and less quantization" vs "more parameters and more quantization" ?
The basic rule of thumb is that more parameters is always better, with diminishing returns as you get down to 2-3 bits per parameter. This is purely based on model quality, not inference speed.
UD-IQ3-XXS is only 13.1GB, which might outperform both in both intelligence and certainly speed (only 3B activated): https://huggingface.co/unsloth/Qwen3.5-35B-A3B-GGUF
To accommodate cache you will need to offload a few feed-forward layers to the CPU. Will still be quite fast.
Edit: Actually 27B does a little better than 35B on most benchmarks- 35B will still be much faster.
9B will be faster, however.