As TW says, pre unit looks a lot better as it fills the frame. Especially when not using a stock primary case cos then you can pretty much space out the engine and box at will. If using a stock primary case, the long ones are better.
This has functional benefits too as the distance twixt engine and box in the featherbed also helps get the best balance between gearbox output sprocket location at the rear (for good chain run, best chain life, etc), and engine position at the front (for handling).
But, unless you’re using after market strong cases etc, the unit design is much stronger both in terms of the crank case and gearbox case.
As to whether the extra strength is required is of course largely dictated by the bikes state of tune and intended use. A mildly tuned road going 650 / 750 used as a ‘classic’ is highly unlikely to need the extra strength.
If using a unit motor, a skilled and careful builder will still get good handling and chain life. If the builder doesn’t possess the design skill, he / she should buy plates from someone like Dave Degens at Dresda.