EMUs are way more efficient than loco plus cars sets. For one, EMUs are lighter than an obese loco, combined with equally obese cars. That means better accelleration with an EMU, and generally lower operation costs.

Looking at weights: A 6-car KISS by Stadler weights (empty) around 300 metric tons. It is rated at 4000 kW continuous power, and 6000 kW short term output. The tractive force of 400 kN gives it an accelleration of 1.1 m/s2. In the Zürich S-Bahn configuration it offers 535 seats plus space for some 800 standees (at 4 people per m2). In order to get those people on and off the train, 12 double-width doors are available over the whole train length of 150 m. If you need a bit better accelleration, think about a 300 m long train with 3 4-car KISSes (for example in the configuration of the BLS MUTZ), and you get 1200 kN tractive force for 1500 seated plus another 1500 standing passengers. Such a train is almost at line speed when the last car has left the stop.

Now, compare that with ACE46. Actually, it is not as obese as I thought, with 100 metric tons, 5000 kW continuous output and 6000 kW short term. The tractive force is 320 kN. The current bi-level cars weigh about 50 metric tons, and have approximately the same capacity as a KISS car. That said, we can assume that in order to carry the same number of people as a 6-car KISS, you will need 6 current bi-levels.

Make the calculation, and you see that the current bi-level cars weigh about the same as a 6-car KISS, but have no traction (yet). So, add the additional 100 t of the ACS46. This leads to an accelleration of something around 0.6 m/s2 (altough regearing to 160 km/h maximum speed would get that value up again.

If I remember correctly, the current bi-levels have single-width doors, which means that it takes at least twice as long to get people on and off the train, increasing the dwelling time at the stops.

All together, the commercial speed of an EMU (of the KISS type) will be significantly higher, and it may well be that in order to maintain a certain operation pattern, fewer trains are needed (meaning a lower investment).

In other words, you get a much better deal using EMUs than loco-hauled stock.