It really depends on what you are sawing and in what direction. Or in short; what size the particles are. If large strips of wood are sheared off, they can easily block the D27 hose, since it's frontal area (seen from the saw side) provides a support for these particles to block the hose. Design wise that could have been different on the side of the saw, but that would also block most of the D36 hose advantages.
I do have a D36 hose, and I mainly use it with the DHL 65 planer. But depending on the material, I also use it on the TS 55. But it really depends on the material, and the direction (relative to the grain) of the cut.
On the TS55 vs TS75; Weight & size. Effective maximum cut length is less for same rail size. If you try to cut out a square piece in a plate (so max saw depth), the blade will stick out more, etc.
Nikon's and Canon's latest batch of super telephoto lenses are more expensive, mainly because they used more expensive, lighter glass. Brompton charges €700 more for their folding bikes if you select the Titanium frame version that's 700 gram (1.5 lbs) lighter than the standard steel version.
In-store, I've seen more people deciding to get the 55 instead of the 75 that they had their mind set on earlier than vice versa.
If 98% of the cuts can be made with the 55, you might get away by doing double sided cuts for the remaining 2%. If you cut so much that you have a lot of these... buy both [tongue]