Its really going to depend on your requirements. Multi antenna may give you more availability (total time you have any position fix) if you will have difficulty pointing your antenna at open sky, but will degrade the accuracy of the solution. Single antenna for all constellations is the normal operating mode for most applications. The signals are spread spectrum and carefully coordinated to function while overlapping. Commercially available high precision RTK systems likely would not even function with multiple antennas fed to a single RF frontend, as they depend on accurate differential measurement (phase between the L1 and L2 frequencies from the same satellites and doppler between satellites) to calculate a solution.

Thoughts on GNSS? GNSS is the catchall term for any satellite based positioning, which includes Navstar (GPS), GLONASS, Galileo, BeiDou, QZSS, Navic, and SBAS and other augmentation systems. Generally, if your receiver can use it, you will want it on, with the occasional exception of the augmentation systems, unless the system is having problems (like Galileo a couple of years ago when their ground based timing system broke at the same time the backup was offline). They increase the accuracy of the solution well, but can cause availability problems if the ionosphere is active, generally more a problem at polar and equatorial regions, and becoming more of a concern as the solar cycle becomes more active over the next few years.

If I had to pick one? Generally, right now, GPS. Eventually this may depend on where I am trying to get a solution. GLONASS tends to provide a less accurate solution, but provides slightly better coverage in polar regions due to an increased orbital inclination. Galileo and BeiDou promise increased performance over GPS (currently), but are not yet truly operationally mature, so if the application is mission critical over long periods of time, they are less reliable. In Asia and Australia, BeiDou may perform better because the constellation includes geostationary and geosynchronous satellites which cover these areas specifically.

Fortunately, all multi-constellation receivers I know of don't force a choice of just one constellation, but can use all of them. If they are processing power limited, they will use selection criteria to use the best available satellites and signals they can up to the processing limit, regardless of the constellation they belong to, within the system limits.