I looked at the specs and saw the T-22 (the ancestors to the T-28) and the GT-30 and TT-38 (ancestors to the GT-18). Here are a few considerations:

– In the older versions, the backpack option used some valuable space; I though it made the pack bigger and heavier. So if you don't plan to use the backpack, keep it simple.

– The fabric is the same, but unless they changed something very recently, the T series (T-28 and T-42 rear pannier) use smaller zippers than the GT series (GT-18 and GT-54 rear pannier). Mind you I have a set of T-42 panniers I have used and abused for 7 years before I retired them last year (IOW they are very very good), but if you overstuff and overfill your bags, the zippers on the GT-18 and GT-54 are more bullet proof.

– I have no experience with their inside liner bag. It seems to be a good addition, especially where you place clothing, but I don't know if I would find it too bulky in actual use.

– Another option: I have neither of these bags. I tour with either GT-54 or TT-84 bags on the rear rack and T-42 bags on the front rack. By themselves on the rear rack, the T-42 are very good bags for commuting, shopping, dayrides and even short tours; for longer tours they become my front panniers.