Having used and/or owned both the TS55, TS75, DF500 and DF700, I agree the weight difference between the TS55 and TS75 is marginal because you are sliding the saw most of the time on the track. Unless the price difference between the saws is a key factor, the TS75 is the preferred choice.
I disagree, however, with the suggestion that the DF700 is as easy to use and control as the DF500. The weight difference between a DF700 and a DF500 is larger than that between the TS's, for one thing. And in use, we are holding and not sliding the machine.
For a small batch of mortises, the heavier weight in a DF700 may not matter (esp. if you have stronger arms), but easily we can be cutting hundreds of mortises, and the cumulative effect is a different story. Of course, if the longer and larger dominoes are what you need, you have no options but to get the DF700.
If double or twin dominoes can meet the joinery needs, even if the price difference is not a factor, I would not want to "abuse" my arms (holding, positioning, and pushing) day in and day out for no added benefits. I find the larger DF700 more straining to use than the DF500, a point also shared by at least two Festool-trained employees at my local store (both are taller and have stronger arms than I). I do not find the higher position of the handle (DF700) a benefit in use vs the handle on the DF500 that is more or less in line with the centre of the cutter.
It is also easier to handle narrower and smaller pieces with the DF500 than the DF700 on some occasions -- trying positioning the DF500 vs DF700 using the intersecting lines technique, and you will understand my point.
Having built tables, cabinets, ladders (and working on chairs now) with no plans for making any exterior entrance doors, I can't foresee benefiting anything from having a DF700 in my work. If a circular saw does the job, why get a wormgear saw is my usual reasoning. (For the record, I can afford owning BOTH the DF500 and DF700 if I want to, as some other members here do.)