6 speed is a far better design for sure. The advantages are so numerous that I'm sure the only reason people still buy straight cut boxes is for the 'look at me' noise.
A good late model 6 speed box will be cheaper than a 5 speed with decent aftermarket gears and the necessary forks etc.
With a 6 speed you get a front LSD thrown in for free.
Option of DCCD for fug all extra.
6 Speed is NOT 40 KG heavier, compare 5 speed with front LSD and DCCD and the difference is more like 30KG. You can easily lose 5 KG off the flywheel anyway.
There are many reasons other than gear strength that 5 speeds fail. Dog boxes get stuck in gear, stuck in neutral, jump out of gear, get 2 gears at once, break output shafts, break transfer gears, break front pinions, etc.
The shift linkage on a 6 speed is all supported on linear bearings, if it's notchy it probably has the wrong oil.
6 speed has a sump, filter, oil pump, pressure fed bearings, optional oil cooler. 5 speed only has splash fed bushes on 1 & 2.
As for the old 6MT can cop it at the drags. Bullshit. Get the right one and it'll do 126 MPH in 4th @8000 R/min.
Having said all the above I still use a 5 speed dog drive in my competition car, because that's where they are best. Cars that do less than 2000 Km a year where weight and clutchless shift is of more importance than anything else.