Higher frequency does allow for a smaller inductor at the same current, but does not necessarily mean better transient response. The transient response is a function of how the control loop is tuned. The switching period is a hard lower bound on transient response, but in reality the control loops are tuned to that pulses average out and therefore they respond slower than that. 20x the pulse repetition period would not be unusual.
Your statement of paying a price for higher frequency in low ESR caps also doesn't make sense. You'd be using low ESR caps anyway in most cases. Even if the control loop doesn't require the low ESR, the ripple current usually does. Caps that aren't specifically low ESR usually can't handle the ripple current at the output of a switching power supply. Note that higher frequency actually reduces this ripple current.
It is also not true that higher frequency implies lower efficiency. At some point it does because the switch can't transition between off and on instantly, but there is a lot of room above 150 kHz before that becomes a dominating factor. 150 kHz is a rather low frequency for a integrated switching chip nowadays.
If low transient excursions are important to you, put a lot of capacitance on the output. However, make sure your type of switcher is OK with that. Depending on the type of control scheme, some require a little ESR on the output. One way to deal with that is to put a little resistance in series with the output before the capacitors, like 50 mΩ. See the datasheet. That will satisfy the control requirements and then you can put as much capacitance afterwards as you want. This allows you to trade off transient excursions for a little overall regulation.
Overall, you need the read the datasheet for any switcher chip very carefully. Make sure to satisfy all conditions. There are various different control schemes, so there is no universal answer. As always, the datasheet is the real guide.
Current capability (which will depend on temperature), reverse recovery time, forward voltage at the expected currents, reverse leakage (heavily dependent on temperature). PIV (Peak Inverse Voltage) rating.
Generally you'd use a Schottky diode for 50kHz unless it was relatively high voltage, then you'd use an ultra-fast recovery silicon diode.
The considerations are similar regardless of voltage or current, but the parts will be quite different for a 5V/1A circuit vs. a 500V/0.1A or 2V/100A circuit (in the latter case, you'd probably want to use a synchronous rectifier and eschew the diode entirely).
Best Answer
The basic deal is that the size of the magnetic circuit reduces as the frequency increases as it gets "recharged" more often so it doesn't need to be as big. Obviously there will be a trade-off between cost of material and cost of miniaturisation and there will be a point at which the sum of both is a minimum. This is what the designer will aim for.
A transient load will cause the voltage to dip. High frequency switching allows rapid correction as the interval between pulses is shorter.
The reason SMPS is efficient is that the switcher is either fully off (no current so VI is zero) or fully on (high current but low voltage so VI is still low). In contrast a linear voltage regulator will be partially on, acting as a resistor and wasting power as heat.
The problem is that in going from off to on or vice-verse there is a short time that the switcher is in transition and relatively high power is dissipated during the transition. The transition time becomes a higher percentage of the duty cycle the higher the switching frequency. Therefore the transistor losses become higher too.
This also has to be factored into the design mix to find the best balance between cost and performance.
I can't help you with this at the moment.