How has Hitachi changed over the years?

Very curious about how the wands have changed over the years. I bought one for my wife a few years ago when I was still dating. Wanted to dig deeper into it. Looks like the model number is the HV-250R. Apparently there are also HV-260, HV-265, HV-270. What has changed other than the latest version has a model with batteries and they have adjustable speeds? Have they changed physically? Internally? How about the adjustable speed? A few years ago people suggested using a dimmer for high/low intensity. Is the adjustable speed enough? Battery run time? Same externally (attachments, handle)?

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