I find it hard to believe that one shinkansen is going to provide more capacity than two locomotives!
The Shinkansen carries the same number of passengers as a normal locomotive, but it moves faster. You can also add carriages to Shinkansen which also move fast to increase capacity.
Which is better depends on the circumstances. If you have a line with lots of long stretches then the Shinkansen could definitely be helpful. If you take any existing spare carriages and locomotives out, the question boils down to: do you want half of the seat capacity for a single faster train? As 6005 commented the additional carriages makes a huge difference to this decision. If you can't add any carriages to the Shinkansen the lower capacity will be a bigger detriment and I'd say the two locomotives will be easier to get further with.