Let me begin by pointing out some of the obvious differences between HandsOnTable and SpreadJS.
Think about your support options.
SpreadJS is a paid product and you can purchase maintenance with the license that is good for a year. This includes live phone support if you need assistance with the control, something that HandsOnTable doesn't offer.
I also noticed so major differences in formulas. I encourage you to browse through the formulas offered by SpreadJS located here
SpreadJS Formula Demo
and plug those into the HandsOnTable sample here
HandsOnSample enter code here
and you will notice that HandsOn doesnt support basic formula functionality.
SpreadJS has a complement component, ExeclIO, that is a webservice ( only supported in IIS ) that handles Excel workbook conversions and sends the JSON ( ssjson ) back to the client and loaded into the excel sheet. I am not sure how well HandsOn handles Excel conversions and if it does, I don't think the client will be able to handle the load of converting some large workbooks.
If you want to list some more specifics I will be glad to answer.