The difference isn't in the overall reach of the distant ends of the system - the Tube's far ends are about 40 miles apart, BART's about 50, not that much of a difference at that scale, especially if you have no need to get to the ends, and my need to get to Pittsburg is minimal - but in coverage in between. As well say that the SFO-LAX shuttle is better than either because it stretches about 350 miles. But can you get anywhere in between? BART is extremely selective on that, the Tube much less so, and there lies the difference.