Not sure but I've just bought a Kriega rucksack for my trip over to Germany and I know RD has Ducati tank bag and other stuff fitted so it will be interesting to compare. The ventura system appears to be a bout £200 for the sport system with 10ltr capacity, Kriega R35 ltr £100, don't know how much for ducati stuff.
I guess if you want masses of space ventura is the one!
The Kriega stuff is great, very simple design, easy to use and you can get up to 40 litres on the back. I went for the Ventura rack and huuuuuge bag, just because it's less faffy to put on/take off and you can take the contents of the house with you in it
Kriega rucksack worked really well although a bit heavy at first but got used to it. Looking at other systems saw a load of Kriega tail packs which again looked very good. I guess it is what suits you and your pocket driven by the amount to be carried. RD had it all but recommended the DP tail pack but not the tank bag due to it getting in the way, tail pack very small though.