Assuming good land use around a station (I live in Jersey City, which is a good assumption around here), I expect that as frequency changes, the demand for using a station at any given minute creates a parabola.
Case 1: There is very frequent service, more than once every 5 minutes. People move in and out of the station quickly, and crowds are very brief. I recommend using the Spanish solution.
Case 2: Medium service, every 5-15 minutes. Transit is still useful, so people crowd the stations. Building enough space on the platform to prevent overcrowding requires more money. The Spanish Solution might still be ideal.
Case 3: Low service, anything less than every 15 minutes. At this point, the longer your frequency, the lower your demand as people switch to other modes, so you need smaller stations. You don’t need a center platform at this point.
This is why the frequency of service vs. demand for space at train stations forms a parabola.