Well, I might as well complicate matter further...….
(As well as annoy those who hate when it gets all technical!)
First off, In brief/simplified-
but I was wondering how a USB bus powered unit could supply power to a condenser.
You can step up voltage pretty easily actually. The "problem" lies in the rule of electrical circuits. Ohms law basically describes how if you change one of the three basic elements of power/current/load, another is proportionally effected.
In this case it means by stepping up the voltage, current (amps or in this case milliamps) go down.
And in the case of what's available from the common/current USB2 bus, this is already pretty low.
If I recall correctly, I believe the standard USB2 5v supply is capable of supplying around 500 mA, or 1/2 an amp of current. Step up the voltage, and that amperage would fall precipitously.
And it may be of curious interest there is at least one commercially available major brand phantom power supply unit that has the option of power using 2 AA batteries. For 48v phantom power. Go figure!
In practice, basing this on similar electronic technology I am far more familiar with...…
I suspect what actually happens is if a load draws more than that power can supply, the voltage falls off as a result. Most solid state devices and microprocessors have a "window" in which they will operate as designed. Drop below that and they can malfunction, or cease to function.
I'll spare you the explanation of how that works......
Not intending in any way to detract from Jon's suggestion, it
is something to be aware of-but in reality, some interfaces likely have more efficient phantom supply circuits than others, and some condenser microphones likely have higher demands than others. So I would conclude that probably- some combinations work fine, while others... not so much. And as he suggests, a serious user might want to consider this element when shopping.
BUT- you might also consider this:
All "affordable level" interfaces I am aware of that have A/C power supplies, use wall wart/transformers that step the 110v AC down to (typically) 12v DC. The MBOX 3Pro is run on 9v DC power supply. Uh-huh.
The big difference here is that the amperage of those supplies would jump up by factor of 4 to possibly 6 times.
You should also be aware, there are stand alone units that insert the phantom power that are popular substitutes for users of interfaces that either don't have any phantom power supply, or have insufficient power.
Affordable units are marketed by companies such as Radial, and ART.
Then there are the more pro level power supply units. Starting in the $250-300 range (and up, of course) you can get highly conditioned power and features like "power ramp up" when powered on. These are the type often supplied with high end/pro mics, and supply "highly regulated" power.
I know that doesn't help with your shopping decision, but at least you will have a good excuse for an extra cocktail while contemplating your choices!
As far as specifics go, I'd like to know how they both work with Mixcraft. How do their preamps compare for noise and clean output levels? My current setup seems to need a lot of gain to get a decent level.
I am semi-retired and don't even have the proverbial "I'm living on a fixed income" that many seniors like to complain about. 
And to make matters worse, I live in a state that loves to make it tough on self-employed people. So that means budget new and also used gear and possibly trades.
and presumably reduces my viable options.
