Sutro is the stronger fit for owners who want more chemistry guidance and do not mind reagent cartridges. WaterGuru is attractive for a simpler automated-monitoring workflow.
Measurement philosophy
Sutro uses a reagent-based chemistry system designed to generate ongoing pool/spa measurements. WaterGuru emphasizes automated recurring monitoring from a skimmer-based device.
The more complex the chemistry system, the more important cartridge cost and maintenance become.
Recurring cost
Both products require ongoing consumables, so hardware price alone is an incomplete comparison.
Estimate two or three years of ownership before deciding which system is cheaper for your pool-maintenance style.
Which owner should choose which
Choose Sutro if detailed routine chemistry automation is the priority. Choose WaterGuru if convenience and a simpler monitoring experience matter more.
Either way, keep a manual test method for cross-checking.