I also have a T3i and the two lenses you mention. Like others have mentioned, you need to think about how your current gear is limiting you, and what kind of photos you want to take.
If you get a new lens, I suggest a wide angle, especially if you do landscapes. Look at the Tokina 11-16. It's EF-S, but it's relatively inexpensive and the image quality is outstanding for the price. There's a new II version, but as far as I can tell it's not visibly better than version I.
You have the 18-250mm focal range covered well. The 55-250 is an outstanding lens for the price. It's sharp, light weight, and has a long reach. The 18-55 is also very good. Do you find either of them to be limiting? For example, if you shoot wildlife a lot, you might want a longer focal length than 250mm. Or if you do low light a lot, you might want something faster than the 18-55. The Sigma 18-35 F/1.8 would be awesome if you want to stick with EF-S but shoot indoors a lot.
People might suggest an inexpensive fast prime. I bought a couple of them: the Canon 50mm f/1.8 and the Rokinon 35mm f/1.4, and honestly I almost never use them. For me, 50mm is too long on a crop body, and the 35mm lens is heavy and manual focus, and I'd rather have the versatility of a zoom. 35mm is too long for star fields, though it would have been nice to be able to use f/1.4 for that.
I use my 11-16 a lot, but still not as much as I'd like. It takes awesome star field photos, but just barely. It would be nice to have more light or less noise to work with. I love the wide angle perspective, but it takes skill to get really great wide angle shots. It's probably me favorite lens, even though I've taken relatively few photos with it so far.
I like shooting the night sky, and I'm in a place where I can do it, and my plan for future gear acquisition is based around that. Once I have a few thousand dollars lying around I want to buy: A full frame body like the 6D or whatever it's successor is (6D II?), the Tokina 16-28 which is the full frame equivalent of my 11-16, and the Tamron 24-70 which is almost as good as the Canon 24-70 II for a lot less money. According to DXOMark, the Tokina 16-28 is *sharper* at it's peak than the Zeiss Distagon T 21mm f/2.8 ZE (tested at f/4 vs f/2.8, respectively)! For less than half the price, I'll take it! And I would keep my T3i and 55-520 for telephoto shots since it's cheaper, smaller, and lighter than a big full frame telephoto.
Think about what you want to do. It sounds like you aren't set on full frame, so you can get good lenses for less money by sticking with EF-S. Going to full frame is a several thousand dollar commitment.
In summary, here are my suggestions for lenses that would offer new possibilities:
Tokina 11-16 for wide angle shots.
Canon 24mm TS-E or another TS-E for architecture, landscapes, etc.
Sigma 18-35 f/1.8 for low light/ indoor shots