As a generalized remark - a lower load resistance will give a smoother (more well-damped) rolloff of the treble, along with loss of extension. Well suited (for example) to MC cartridges with a hot high end. A larger resistance will gradually expose the HF resonance, providing more sparkle at first and more fatigue eventually.
For specifics, you must either get the manufacturer to give you guidance - only they know the details of the transformer's more subtle parasitics - or do the experiment yourself. Unfortunately the quality fo the resistor will influence the results, so you will never find a conclusive answer, but you will likely get close enough for personal satisfaction.
