Haven't had time to frequent the site much of late, but enter and find a very interesting post. Unusual in that I own the 3 bikes on yout list. What are the odds.
I'll try to be of help here.
99 FC501 Husaberg is the lightest of the 3. Mine weighs in at around 235. Typical old style Berg. Turns on a dime and at higher speeds is a bit unsettling. Steering damper helps a lot. Vibrates the most of the 3, which is most annoying at higher speeds. Engine has smooth power, decent low end, and good mid/upper pull. Old style seating kind of keeps you planted in one spot, but is the most comfy for longer trips. Lowest seat height. Suspension is surprisingly good for an older bike. A little fork flex can be felt. Least flywheel of the 3. Clutch, brakes, and shifting are fine. Not a bad bike that is nice to ride on the track or in tighter off road situations, although suspension is a bit soft for track use. Very reliable year for Berg. 90+ MPH.
03 FE501 is much more modern in seating arrangement. Less vibration. Little more power throughout compared to the 99. Unsettling at high desert speeds. Mine has a GPR damper. Weighs in at 255. Lights and E Button have a price to pay. E button is nice although some people have had problems getting there bikes to start on the button. Mine always starts fine. Turns great in tighter stuff. Feels lighter than it is. Suspension is nice off road, although forks transfer a spike on square edge bumps. Too soft for moto. Excellent hydraulic clutch, wave rotor brakes are also exellent. Shifts ok. Great all arounder. 90+ MPH.
00 TC610 that has had the TE 6 speed added to it. Now this may not be an accurate comparison to a TE 610. Without the lighting and with a lightweight single exhaust, the TC is much lighter than the TE. Mine weighs in at 255 with a small Acerbis headlight. A stock TE is pushing 270. TC also has a more agressive cam and exhaust. Stock TE has 42HP. TC has close to 50 as do the 2 Bergs. I once owned a TE610 and power was too much at lower engine speeds. TC cam smooths out the low end making it much more manageable. Top end revs are unreal for a 610. Husky is the fastest of the 3 in a drag race and even with the TC cam, has the most low end. Also has the most flywheel. Vibrates very little at low revs and feels light. Rev the engine and the vibration gets annoying and the flywheel mass makes the bike feel heavy. Likes to be short shifted. Suspension is fine for off road although spikes come through the front end on square edged bumps. TE has softer suspension and is in dire need of heavier springs. Clutch and brakes are fine. Shifts buttery smooth. Chassis is super solid. Instills confidence. Seating is more modern. Very little in the way of headshake, yet still turns ok. Magic here or something. Stone reliable. Starts easy. Nice all arounder. A little slower on top end than the Bergs due to stock TC gearing.
Hope this helps.
Dan