I updated from the MkI to the MkII simply because the opportunity arose. Whilst the MkI was an L lens, it was the worst of the three original TS-E's in terms of optical performance. That fact is probably no surprise as 45mm is the simplest focal length to cater for and 90mm doesn't present the same issues as the wider end of the range where focal length is less that the register distance.
The MkII has several advantages of the MkI in the way the tilt and shift mechanism works....ie, totally independent and not mutually exclusive without dismantling the lens (as was required on the MkI). It's also optically superior but this will only be really evident at the larger apertures and when getting near to full shift. If you're going to be shooting at f/8 with a little bit of tilt then the visible benefits will be marginal. If you intend to shoot at f3.5 and some shift (ie, interiors) then you'll be seeing more gain for your outlay. The MkII has more shift capability but this can lead to its own problems if it's not handled correctly and especially with the closer foreground objects if you intend to stitch shots. There is less distortion, less vignetting and the corners remain close to the sharpness of the centre region. Ultimate sharpness across the frame is something of a mute point in reality as the TS-E design requires a degree of field curvature and this can be seen in architectural shots when set up perpendicular to a flat faced subject (ie, the wall of a building)
Is it worth the price difference? It cost me about £800 to make the change and, as an amateur, I don't have the need to see a return on the lens other than the satisfaction in knowing that it's the better of the two options even if there are occasions when the improvement is barely tangible.....you criteria may be very different.
Just a note on the 24 v 17 debate. I went for the 24 because of the difficulty filtering the 17 (at the time). By the time that I was seriously looking at adding the 17mm version, Zeiss popped up with their 15/2.8 which not only works with Lee's 100mm filters but can take regular screw-ins as well.
Bob