G100 and the spX, as the others have said, are essentially the same. The G100 has improved interior from Gulfstream's completions experience. And the later serial #'s have two small CRT's for engine indications rather than the analog stack. The CG issues on airplanes with an APU (most do) are dependant on whether or not the ER tanks, and ballast weight are installed. If the tanks are in, the 49 lb. ballast weight that "pins" in the NG wheel well should be in...and only if you're going to fuel to "full bag", otherwise you can run into fwd CG issues. Conversely, if you need the extra fuel of the ER tanks and the ballast weight is not used, you can experience aft CG issues.... Non APU airplanes are a fwd CG challenge most of the time with lighter fuel loads.
One of the shortcomings of the tanks is that they greatly reduce the aft baggage capacity. Another is that it is not a simple task to R&R the tanks. Most operators that use them just leave them in. For our ops we found that baggage space was more important than an extra 30 min. of fuel.
Of course there's the infamous flap/slat issues on these birds. Look for an airplane with C/W SB's that improves the flap/slat PDU's and flex shafts.
Airbrake actuators can be an asymetrical PITA as well. Ensure that the actuators have been properly bled. The bleed procedure has been revised following "discovery" on S/N 152....
Brake swivels on the MLG trailing link pivot can be leaky and problematic if not properly maintained as well.
On the fun side the airplane is the fastest in its class with a .875/353 Mmo/Vmo, and can run 490+ TAS in the mid 30's at mid-light wgt. It is most comfortable at FL390-400 where at max cruise it'll fly at .82 to .86 depending on your weight. Fuel burn swag/hr is 2000/1500/1500/1400 thereafter. You can stretch range by flying the AOA at a best L/D of .29, where .78 -.80 gets it IIRC. It has good pitch feel, and moderate roll force, and is very solid handling overall. It has excellent stall characteristics with no roll tendency. It has no shaker or pusher and in the stall if you hold the yoke to your gut she just mushes, flys a little, and mushes...very docile. Add a little power and she'll recover very easily.
Standard fuel is 8,692 lbs. and the ER tanks add another 100 gal. for a total of 9,365 lbs. While it has better runway numbers than a LR-60, it's not a short field performer. With a BOW of around 14.5k (sorry but I just can't pull the exact BOW of the last one that I flew out of my brain tonight) you could go out of Hilton Head at 22k on a standard day. On a more realistic 30C day the BFL would be right at the limit at 21k. Those numbers are straight from tab data.
There is alot of "very good" in and about these airplanes, but as mentioned long distance cabin comfort isn't one of them. The G100/spX is the WW cabin after a strong dose of steroids.