I own a MKI Multistrada and have ridden the 1200 a few times, and unless I have missed something, the Multistrada will be the most 'sporting' of the three.  If you want to keep up with R1s and GSX-Rs, but still be able to tour and commute, the Multistrada is a great choice.  However, I wouldn't fancy taking one off road on anything other than a gravel track.  The electronic suspension is OK, probably not worth the premium in cost and complexity (only rebound and compression damping is adjustable on the front, pre-load, rebound and compression on the rear).  The big downside is price.

The 1200GS is probably the best big "do it all" bike there is, it can be taken off-road, has great touring potential and a whole industry supporting it with farkles and sparkles.  More expensive than the Tiger 800 it is also likely to be a heavier.  I've only ridden a GS once, but the telelever front end would take some getting used to, as would the sheer size and weight of the beast.

We'll have to see how the Tiger rides, but my guess is that in road guise and on the road it will be a better handling bike than the GS, it should certainly be substantially cheaper than the other two, and may also be lighter than both (certainly lighter than the GS).  For long distance work, the smoothness of the triple should be less tiring than a twin.  Since many 1200GSes are bought for road-based touring, I think there is a very real chance the Tiger could eat into the sales of the larger GS, if it does make 100bhp and weigh in around 185-190kg dry.

I don't do BMWs, but did have a Tiger 1050, and it runs with the R1200GS just fine.  Current ride is the MTS 1200 Standard.  It is a road bike just like the Tiger 1050, but it is lighter, and more HP (a WOW! combination).  DTC is required on the Duc unless you ride it like a girl.  The Duc is nicer 2up ride (according to my SO) than the Tiger 1050, suspension is on soft/medium side (I am 180 lbs).

I am interested in the 800, but don't expect it to replace the Duc.  It will never out handle it, or keep up with the HP, but I expect it will be handier off road.  I do not go off road with the Duc, other than firelanes or gravel roads.

I'm with Grey Dragon on this: I've been riding since 1968 and even so most bikes over 500cc have more capability than I have skill/confidence. I had a 2 hour test ride on a MS1200 and was blown over by the electronic torys and traction control, but simply don't get the need for so much power on a road bike. It is also, in the UK, stupidly expensive. They wanted almost £12K plus my 2008 Tiger to do a deal....
The Tiger 1050 is, so far, the best bike I've owned, but it is a road only machine. I want to go onto some of the tracks and green lanes here and the 8000XC might be the answer, although the clearance between front wheel and lower musguard concerns me. Will it do mud? If it is also road capable then it might displace the big cat, but possibly not. 

Not sure that the Multi is an "all-rounder".
It 'can' go offroad - not sure how it would do.
Dual sport tires can be fitted on it - but not true knobs...let alone a small get-off/layover would prove to be
VEEEEEEEEEEEEEERRRRRRRRRRRRRRRYYYYYYYYY Expensive.!

The true all-rounders are the units that have proven themselves for years:

BMW's GS / GSA
TRIUMPH Pre '07 Tigers
KTM 950/990 Adventures
& others.

They both do well on the tar and on the outback. Some better than others.

I am in high hopes that the XC is right in there with that elite list.!

Absolutely agree, although not sure when we are likely to get onto a test ride of the Tiger down here. I had hoped to have a new/additional ride by mid-March for a trip I am planning, but that may not happen. Unfortunately I am not prepared to take a leap of faith that many have and order sight unseen/unridden.

Thanks for all the great comments - most insightful and useful. As the new bike will be a commute as well as a tourer, and less of an offroader, I have probably crossed off the 1200GS. The expense of the MTS is also of a concern, although the 800 may get close with a few options added - we will have to wait and see!