Lsp is about the efficiency of engine(burning fuel). In space high lsp is more important than high trust, as from physics high lsp => high velocity, no matter how much time you need to get that velocity. For example: if your lsp is very high and trust is very low, you will get speed slowly, but fuel will be enough for long time and if your trust is high and lsp is low you will consume your fuel quickly but the total velocity will be lower as it burns rapidly. At surface of planets as long you haven't a chieved elliptic/circular orbit, the gravity will drag you down, especially at the launch. Here trust is MORE significant, because you need to get height quickly otherwise the gravity's affect on you will be higher. So your first stages should have high trust and "space" engines need only high lsp, while orbiting you always will have enough time to do maneuvers
