I had both as company vans, the T4 showed rust on the front rear arches and around the door steps after a few years (I do look after my company vehicles) the glow plugs went on all the fleet at around 70 to 80k and belched out smoke, not bad on fuel though. 185k and 6 years old when my next van arrived
The T5 had more comfort and drove better, didn't notice any issues with rust and had no issues with electronics in the fleet.
Did have a new set of rear springs on mine at approx. 20k as the originals broke.
The one thing that did keep braking where the rear light bulbs, don't know why, but I ended carrying spares in the end, none of the others had this problem. 155k and 5 1/2 years old when my next van arrived
Now have a Vauxhall Vivaro and must say, I have had no issues at all with my van, just shy of 50k and 2 years old and not even a light bulb, very good on fuel and tyres too.
Not so comfy drivers seat though
Hope this helps