I can't comment on the 400, as I've never ridden one, but I
love my 650. I've had it for two weeks now, and I've already ridden a two day, 600 mile (960km) trip across the Cascade Mountains (via Stevens Pass) and back, and a day long, 330 mile (528km) trip to the Olympic Mountains (Hurricane Ridge in Olympic National Park).
I rode scooters back in the '80s but this is a whole new ballgame (see my post
How fast will it go in this forum). Plenty of power for overtaking and passing, even going up hill with two aboard.
The 650 weighs 607 pounds with a full tank of fuel, compared to about 800 pounds for a fulldress Gold Wing, so if you've handled the GW comfortably the B
urgman 650 should be a walk in the park. Maximum Gross Weight is 999 pounds, so carrying capacity with full fuel is 392 pounds of rider, passenger, and cargo.
I don't know about your GW, but the current model has a seat height of 29.1 inches, compared to 29.5 inches on the AN650K, so the big Burgman is less than half an inch higher. I don't know if that would be important for you. I'm 6' 5" so I can't really relate, but 4/10ths of an inch seems irrelevant. (The Burgman 400 seat height is 27.4 inches.)
The 650's tires are large for a scooter, 15" in front and 14" in the rear (the 400 has 13 inchers front and back). While not as large as the GW's, they give me a confident feel on the road -- much more so than my old scooters did (Honda Elite 125 and 250).
The Burgman 650 has some really great features that make it a good bike both around town and on a road trip. The continuously variable automatic transmission means the bike will accellerate briskly from a stop even with a heavy load, then run smoothly without any noticable shifts all the way up to freeway speed. At any combination of speed and load it is always in its optimum RPM range.
Having both brakes on the handlebars (no clutch, so rear brake goes on the left bar) can take some getting used to, but is really nice once you get used to it. The absence of a clutch, combined with the CVT, means no worry about rollback or stalling when starting out on a hill, and with both brakes in your hands both feet are free for holding up the bike.
Unless you add an aftermarket trunk the Burgman doesn't have the storage space of a Gold Wing, but 56 liters in the trunk is more than most motorcycles and scooters offer. It's enough to hold two fullface helmets and a couple of light windbreakers, or more baggage if you use the helmet hanger instead of putting your helmets in the trunk.
The best advice I can give, if you're really serious about getting a scooter, is to go to a dealer and look at what's available. Ask for a test ride if the dealer has a demo model. If the 650 is comfortable for you to ride around the block a few times, then I think you'll find it to be just what you're looking for.
In the mean time, click here for official specs on the 650:
http://www.suzukicycles.com/Products/AN650K4/Specs/Default.aspx
A menu to other models is also there.
HTH.