[PNF]

umm.... 70-200 f2.8 is literally 2x the weight..

uber, it really depends on what kind of photography you do.. if you do sports a lot and need that extra stop to increase your shutter speed, by all means, go for f2.8. But if you dont have a FF or a 1D or 1Ds body.. and its 21 pt focus point/sensor.. i dont know how much "sports photography" you do..

f4 and f2.8 are both great lens.. and realistcally speaking, normal people just dont carry f2.8 IS around every time they take them out.. I know I wouldnt.

Sure I'd take it out for a troll in NYC, but I wouldnt take it on a troll every time I bring out my camera gear..

For me, 70-200mm is to fill my void of long range/tele photography and I'd try to carry the lens with me all the time if possible.. with f2.8, I cant do that, because I dont plan to bring 6-7 lb of gear every time I go out (with a 17-55mm everyday lens + flash + misc gear/filters/cleaners etc...)

Think hard on what you're using the lens for at this point.

If f2.8 is only say 0.5 lb or even 1 lb heavier.. thats fine... but at 1.7 lb heavier... most pros use a monopod for that much weight with a 1D body.

oh well, do let us know your final decision!!

(PS, I wouldnt get a used lens... just me, since you never know what could have happened to it or how previous owner used it)

Get what you can afford and what you need... dont "over-spend" on a lens that you probably use 40% of the time. Get the 24-70mm f2.8 (if you have FF) or 17-55mm (if you have a crop body). + a 50mm f1.4.. (for 50mm, i really do think f1.4 is more than sufficient and very well built for its price)... and then a 35, 105 or 85mm f1.4.. they just rock! )