The old/new GS has the same engine as my Scarver.
There is a new/old Scarver too.
Old=one spark plug, new=two spark plugs.
When it needs to adjust valve clearance, it is easy to remove a shim without remove the camshaft on the old one spark engine.
On the new two spark there is a cap covering the shims so you (not you personally, Duchess!) have to remove the camshaft, and that means you might get some trouble to time up the engine after the job.
But don't worry about it if you want your dealer to adjust the valve clearance for you - they know how to do it!
The first F650's har two carburators, the first injected F650's is one spark, the last has two spark. All have chain. The only F650 with a belt is the Scarver.
The factory lowered GS has a lower seat then the Scarver.
The rest of the bikes are different from my belted Scarver, but a few parts fits the injected F650's and the Scarver, like some parts on the handle bar area, some fuses, some light bulbs, the top box (if you change the luggage rack on the Scarver) and the blinker fluid. (

blinker fluid!

)
Some of the one sparked F650's, GS or not, surge - they have trouble making the engine work smooth all the time. Some of them, not all - mine are OK.
Because the 650 has one big room inside the engine, 650ccm, they figured it needs two sparks to make all the petrol ''burn''.
Some owners of the old boxers R100 gave the engine one more spark per cylinder because of the big room (500ccm) and only one spark. The old boxers works fine with just one spark plug per cylinder, but it seems to work even better with two spark plugs per cylinder.
So:
Injected;
one spark = more easy to adjust valve clearance at home.
two spark = perhaps feels better to ride because it ''burns'' the petrol better inside the engine.
Carburated, one spark = old. (-Is there an carburated GS? The carburated are Funduro and Strada?)

Liv.
(Alomost all modern motorbikes and cars has injection OR carburator(s). Injected engines needs less petrol, and they often have a catalytic converter. Catalytic converter = cat - it cleans the exhaust before it gets out in the air.
...and there is diesel, but I feel sure you are not going to find an BMW F650 with a diesel engine! Diesel = no carburator, old or new.)