I don't know what VTX1800 you're looking at, but the MSRP is $13,399 according to
www.honda.com The Silverado Road-Star retails for about the same, according to Yamaha motors website.
If you're able to get them for $8000 I'll but every single brand new VTX1800, and Roadstar you can get your hands on, and I'll give you $10,000 American for them. Maybe you're talking in pounds sterling or Euro's or something?
As for picking a bike, most of the big ones are about the same, it all comes down to matters of personal preference. So whatever feels the best, I'd say sit on it.
Of course I'm generalizing from a single example here, but everyone does that. At least I do.
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