The main differences in Brushed Alternators and Brushless Alternators is the way they excite the internal stator. Typically larger generators are all brushed as they can provide faster load output then a brushless alternator can but brushed systems can require maintenance to the alternator by having to change the brushes. Brushless systems do not have this maintenance option available and when the unit dies it cannot be repaired as the cost would be more then replacing one.
Either way, IMD states that either their brushed or brushless alternators will maintain an appropriate level of Hz output where you will get 5% or less THD (Total Harmonic Distortion) which is comparable to or better then standard utility power.
AVR gives you a little better/tighter control over the voltage output, usually +/- 1%, versus a standard capacitor which may vary a little more like standard utility power.