What lenses do you have for the 550D, and what are you intending to get with the FF camera?
Just that speaking as a recent "upgrader" from 400D to 6D, I am not sure that you will see much benefit from moving to full frame for wide angle landscape shots unless you spend a lot of money on heavy and expensive lenses.
My EF-S 15-85mm gives me a very good wide angle IQ on the 400D, while the closest to equivalent on FF, the 24-105L has more distortion and vignetting at the wide end, while not being as long. Correction of the distortion in PP reduces the wide angle a bit. The EF-S 10-22 is excellent if I wished to get wider on the crop camera, and the IQ of the 17-40L if I buy it may not match this.
If you want to get shallow DOF in low light, then that is where the FF will excel. My wide angle / landscape shots are generally stopped down, and I believe will not see so much benefit with FF. My "old" 400D will still see some service in this role.