Not sure - I've had two fully laden panniers on the back and a bag on top of those, to the point that lifting it up the few steps out of the garden was awkward, but once on the saddle didn't really notice the difference (probably slower and harder going uphill/up-wind but I wasn't pushing hard enough to notice, just pootling)
However, when I've done 10+miles on the local country roads I find I can't always get my wrists comfy - not often, just every now and then - combination of poor road surfaces, 70PSI tyres and a flat bar.
If I was buying for a tour, I'd get a drop barred bike, but I've never had drop bars and this bike was bought as a commuter for a ~3mile commute - and has done this admirably.
(Had a quick look and the vienna look good - I stand by my previous statement - get yourself a ride on both and see which just clicks or which excites you more)