Whast the major difference, if any?

I'm assuming the Bosch will be more reliable than the Lucas/Sagem system, and are they interchangeable?

from the plenum down they are the same, except some plug connectors, also, i think, the oil filter connector on the later engines has a 45 degree attachment, i have removed this and the oil filter slots straight on. i do have an oil leak from that area tho. i would suggest if your putting a newer block in a older car you use the older front case as this is designed for the straight down oil filter that wont interfere with the thermostat on the rad!

the injection and plenum obviously will fit but i dont think it is compatible with the old loom and ecu!

The Gems is easier to work on because of the shape of the Plenum,and because of the poor AFM's on the Thor ones I'd say the Gems is more reliable.Saying that,both are very reliable and for their time clever systems.The Thor engines use a different spoke pattern for the cam sprocket but the same sensor.
Most Gems/Bosch problems result from abuse/no maintenance or having to tolerate a cr-p,badly tuned LPG system.