It all depends on what your long term objective is. If all you want to do is either print and cut; or cut iron on t shirt material then one of the 24" models would do you fine.
If you ever plan on cutting vinyl graphics for cars, boats, truck, banners etc.. then you will need a larger plotter. A 30" plotter would do well for this type of application plus you could still use it for the same applications you would use the 24" plotter for.
If you think you will ever want a large format printer (car wraps, murals, banners), say 48+" wide then you may want to look at the wider plotters.
Now if I had to limit my decision between the Roland or Graphtec, that would be hard. Both are very nice and capable plotters and have tech support if needed. This will sound pretty simple, but between the two I would probably go with the Roland, only because it seems that replacement cutting blades are less expensive.
If I did not have to limit my decision between the Roland and Graphtec, I would probably get a Summa
