One of the differences is the number of leg sections. The 1545 is a 4-leg section tripod and the 1555 is a 5 leg-section, meaning that when fully collapsed, it will close down smaller.
The 1545 is also about 2" taller. Regarding stability, the 1545 will be slightly more stable for two reasons. One is that it has fewer leg sections and secondly, because it has fewer leg sections, the lower of the sections is slightly larger in diameter.
If it's not a problem that the 1545 is about 3" longer when it's folded up, I'd recommend that one. With a scope, especially at the higher magnification range, the added stability will be nice. You could get away with the smaller 1555, but in some situations you may find it to be just undersized.
Both tripods are considered super compact. If you have any other questions, feel free to call Eagle Optics at 800-289-1132.