I'm not sure the experiment is going to work mainly because the conditions you keep the plant in will be different. Light, humidity etc are all going to come into play here. If you both decide on the same amount of water a day and one of you has a plant in a dryer/warmer place it will obviously die sooner because of lack of water. Also tomato plants needs feeding to thrive.
Just as an experiement though.... I planted up some geraniums the other day. I will carry on as normal with my watering regime but I will do a couple of them with microwaved water, another with filtered and another with tap just to see for myself if there is any change. They are all in a lovely warm conservatory at the moment.
I think it is easier to do the comparison in the same environment than different places because of varying conditions. Just my twopence worth....
Blimey Harrodine, is that right you have a sense of humour :-0 ;-)