Depends on which configuration of the LBO you are talking about. It's the more flexible system between the two mentioned and can expand to fit up to 6 guys without a stove, or 4 with stove and gear.
No experience with the supertarp but I have an LBO with the 3 piece vestibule and the connecting tarp. For 2 people no stove the LBO & Vestibule is awesome. Add in the connecting tarp and it's good for 2 people, a stove and wood or probably 4 people with no stove.
I also have an LBO w/ the 3 piece vestibule. It is very roomy, and would comfortably fit 2 people and gear. I usually just stay in it solo, and it is a palace for one. A hunting buddy of mine has a supertarp, and it is plenty big for one and all the gear you want to throw in it. 2 people and no gear would work, but it would definitely be tight. Adding an annex would give you some additional room, but it would still not be as roomy as the LBO. I really like the supertarp due to its material, packability, and various pitches, but the size advantage goes to the LBO.
I have no experience with the LBO, but I really enjoy the supertarp. I have tried other shelters and tarps and keep coming back to the supertarp. Like stated above it is very versatile and if it is just you, i think you are going to have a hard time finding something that is going to beat it.