It depends a bit what you bring and how much you pack it.
I have filled to capacity the T-42 on weekend outings where I didn't need a tent, but a sleeping bag, but they were in Spring where I need to be prepared for any kind of weather. Case in point, in 2004, I went there with +15 C... and came back the day after at +2 C. I also had a book or two and my camera in the panniers.
In a nutshell, if you are a minimalist and mostly eat in restaurants (or people's places) and sleep in hotels, you will have plenty of room with the T-42. If you are further away from the beaten track, camp, prepare your own cooked food, prefer to bring a few changes of clothes rather than wash daily, then you will prefer the GT-54. In all this I have assumed that your tent and sleeping bag would be attached on top of your rack, and that you might have a handlebar bag for a few minor items (camera, maps...).
As for a practical comparison between both bags, I'll say it's the depth (20 vs 15 cm) that makes the most difference. The T-42s tend to remain flat whereas it's easier to overstuff the GT-54s. For example, I think I can only pack a mummy (narrow sleeping bag) in the T-42, but I can pack almost any type of sleeping bag in the GT-54. I also like the outside pocket of the GT-54 for toiletries, and the large vertical pouch is great for tent poles... or 3-4 extra bottles of water.
As for heel clearance, it is not a problem especially with the new adjustable hooks. Just push the panniers further back.
Assuming you don't have front panniers, you might be OK, depending on your touring style. Unless I'm mistaken, weather ranges from very hot to too hot, so you won't have bulky clothing to carry. Other stuff:
- if you bring cooking gear (stove, pots...), you will probably prefer the GT-54;
- if you go to remote places and need to bring food for a few days at a time, then you'll prefer the GT-54;
- if you travel through civilisation all the time, mostly eat locally and/or with cold food, then I think the T-42 could be enough;
- Tools : most of the tools are heavy, but don't take too much space.