When I built our tandem few years ago i decided to go with Campy Centaur shifters instead of Shimano. I still have a Shimano drive train but use a JTEK shiftmate to make them work together. The reasons for this are as follows:
Shimano has been getting more expensive compared to Campy
Shimano has to be thrown away when it stops working (wears out), Campy is rebuildable and you can buy parts for it.
Campy is lighter weight.
For these same reasons I also have Campy on my single bike.
In general I like the shape of the hoods of Campy better and I like the thumb shifter when shifting down. Last fall I injured my right hand and it is still bothering me. I have been riding with a brace which makes it difficult especially when standing. I decided to put Ultegra Di2 on my single to make it easier to shift and because I wanted Di2 after reading more about it. The Di2 will be installed this week so i do not have any experience with it yet. If there was Di2 triple I would consider it for the tandem. Today I rode my older single which still has Shimano shifters and it seemed to take less force to shift (mainly up shifts) and put less stress on my hands. It is the old 7800 and I don't like the shape of the hoods. Also the front shifting is considerably better than Campy, but that could be partly due to it being a double and having a Shimano 6700 crankset. Still the front shifting is better than my single with Campy and Specialized crankset.
So now I am wondering if it makes sense to consider Ultegra ST-6703 shifters for the tandem. The benefits being less stress on my hand and possibly improved front shifting. If it was not for my hand injury I probably would not be thinking about this.