I've never ridden a ST1100, so I can't comment on that bike. However, the principal shortcomings of the ST1300 are well documented throughout this Forum, and seem to come down to the following......
Uncomfortable OEM Seat
The cheapest fix is the fit a sheepskin cover and set the front on the middle setting, and force the back down onto the lowest setting, which has the effect of flattening the seat. There are quite a number of after market seats available, and penty of discussion on this forum about them.
Noisy Winsdcreen
I found the OEM screen to be very noisy, and like a number of other forum members have fitted a BikeQuip screen. It's much quieter. Other's have fitted other brands of screen.
Hot Crutch Syndrome
I've not experienced this problem on my 2008 bike, and believe that later versions had better heat insulation. A sheapskin seat cover provides a better seal between the tank and seat, and has been reported to minimize the issue.
Pan Weave
Often cited as a big problem, but I've never experienced it, and I understand that it has only been reported at very high speeds. There was a recall to modify the swing-arm pivot to help. I note that the Owner Manual states that 130km/h should not be exceeded when "carrying cargo". If you don't do racetrack speeds on the highways, I do not expect that Pan Weave will be an issue.
Pie Warmer Heat
The two glove boxes do get hot, and I don't use mine to store anything that could be effected by heat. They are an ideal spot to store wallets, cameras, iPod and so on, but I won't put mine in there. I made some measurements and found that on a 30 degree day, temps in the Pie Warmer reached 43 degrees while on the move, and 57 degrees soon after the bike stopped. Who knows what temps would be reached on a hot day! Some owners have fitted insulation around the outside of the compartments, or removed certain panels. It would be nice to get some temperature data before and after these mods, to see how effective they have been.
Low Bars
Fit risers, often cited as the best mod that owners have made. I fitted a set of BikeQuip risers, and the discomfort that I'd experienced in my neck and shoulders after a long day in the saddle has gone. There are may brands of risers available to suit the ST1300.
Touring Farkles
Many of our owners have made additions to their bikes, to better suit their own style and needs. Options have included cruise control or throttle lock, heated grips, GPS, headlight globe upgrades, highway blades, CB/Pillion communication systems, driving lights, sound systems etc etc. What you need is very much a personal choice. There is a wealth of information on this forum to help you choose your farkles.