I have the MKI 70-200mm F2.8 L and have just upgraded to the MKII IS version.
Mine is a 2005 model so 9 years old and as far as I know there is no update in lens design could be wrong.
In fact there is only a very small difference IQ wise to the MKI Non IS and the MKII IS. But the smaller minimum focus distance and IS are a huge huge benefit, and it is very slightly sharper. The Non IS is sharper than the MKI IS too.
I would say go for it. Nothing like an F2.8 zoom the 70-200mm is a staple and one of the best lenses I ever bought. Use it all the time.
Just be careful, its hard work keeping that heavy 2.8 lens sharp at less than the rule of focal length to shutter speed. So at 200mm shoot no less than 1/200, It can be frustrating at times which is why the IS version is so good at 200 you can shoot at 1/30-1/50 with relative ease as long as the subject is stationary.
For the price difference it is a bargain.http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=242&Camera=453&Sample=0&FLI=4&API=0&LensComp=687&CameraComp=453&SampleComp=0&FLIComp=3&APIComp=0
The F4 is a great lens but unless you need a lighter slower lens I would have the 2.8, especially if you are shooting crop, as the F2.8 will give DOF equivalent of F4.5. On FF its a no brainer F2.8 gorgeous subject isolation at all focal lengths. You get great portrait lengths 85mm 135mm and 200mm all at F2.8 which rivals the primes at that aperture.
Enjoy! Remember L lenses regardless of age hold their value so if its not for you move it on and buy another