For business travelers staying in corporate housing San Diego, the question of the optimal mode of transportation can be answered simply: it depends. For a resident who is doing business exclusively within the city limits, San Diego has taxi services galore. Of course, if complete freedom of mobility is desired, a rental car would be best.
San Diego also has the downtown trolley, which has received rave reviews for the pleasantness of its ride and convenience to attractions the residents of San Diego corporate housing might like to take advantage of. If business is being done regionally, the natural assumption would be that much of it would be in Los Angeles. Depending on the budget available, there are numerous heliports in downtown San Diego that can serve as a link between the two cities.
But by far the most economical and relaxing way for the residents of San Diego corporate housing to reach the northern metropolis would be by Amtrak. Since transportation once in Los Angeles is almost entirely car-based, a rental car for the day is almost required, unless the traveler intends to go no farther than the downtown area, in which case a taxi will suffice.