I guess I'm not very good at getting my point across.
I will concede that a 1200 cc Vulcan would burn a little more gas and the insurance cost would be a little more than a 900, but as far as accessories are concerned, there would be ZERO difference. At least, that's the way it is with the Harley 883 and 1200 Sportsters.
If you want mufflers, floorboards, windshields, handlegrips, saddlebags, seats, chrome air cleaners, or whatever.......... if it will fit on a 883 (or 900), then it will fit just the same on a 1200.
And, since the 1200 has considerably more hp than the 883, they can (and do) make the gearing a little taller on the 1200 to make it better suited for highway use without the need to go to a 6th gear in the transmission.
In effect, going to a 1200 instead of a 900 would mean much better performance, greater acceleration (particularly if riding double), easier highway cruising, and still have the same size, weight, and handling characteristics as the current 900. Plus, any accessory that you would want to put on the 1200 would be exactly the same product as what you now put on your 900. For the very slight increase in gas consumption and slight increase in insurance costs, I would choose the 1200 and never look back. Just my 2 cents. Obviously, some are convinced that the 900 is nearly perfect and can't be improved.