I've got both the TT large rack and the Hepco & Becker rack.
The TT large rack is good because it's flat which gives you a few options on how to secure your load of various shapes/sizes, but there is a recommended weight limit, which is 5kg IIRC.
However, I normally run with the H&B setup. It's a strange shape, but has 2 handles built in that can be used to lift/pull the bike, and I normally use the seat side to secure my tool bag (which is a fair weight) to sit on the rear end of the seat, which leaves the rear 'flat' area to secure any lighter misc items, which has included a small tent and roll mat in the past. Same recommended weight limit for the H&B, but I also have the H&B pannier frames that fit quite quickly should I need to use panniers or offer support to the subframe to carry any extra weight.
I think I also have a small TT rack which came on a bike, but never used it in favour of one of the above.