Frank-
As with all track systems, each has its own strengths and weaknesses. Realtrax is designed to be a replacement for traditional tubular track- very easy to setup, no connector pins to lose, and a nice appearance thanks to extra ties and molded roadbed. While the track is durable, the connections become less reliable if the sections are connected and disconnected frequently; you may encounter instances where jumper wires might have to be soldered to the rails to bridge dead spots between tracks. Like Lionel's FasTrack, RealTrax is an unforgiving track system when it comes to minor trackwork or track plan issues (misalignments, gaps between sections, etc.); the tracks either fit together or not at all. Because there is no easy way to create custom cut track sections, you may have to connect one or more small "fitter" pieces of track together (instead of one custom cut section) to close short gaps. Naturally each fitter introduces more track joints, and more track joints can result in more places for electrical connections to fail. Another downside with Realtrax is noise - the hollow cavity under the rails acts as a resonating chamber, so sound-absorbing material is a must to control the "roar" as trains roll along.
MTH's ScaleTrax was designed to compete with Gargraves, Ross, and Atlas O's track system. It is a solid rail system with plastic ties and a T-shaped scale rail reminiscent of Lionel's Super O track system. As an added bonus the center rail is blackened and thinner than the running rails, which gives the appearance of two rail track. The sections connect with copper spring joiners similar to Realtrax, which are very delicate and will weaken as the sections are disassembled and reassembled. Expect to add extra lockons or solder jumper wires to alleviate dead spots, but keep in mind soldering to solid rail track will require lots of flux to ensure a good connection. ScaleTrax, like tubular track is forgiving; MTH even offers flexible piece that can be bent into any shape or curve. Making custom cut sections is also easier with ScaleTrax, but there is no way to reattach the electrical connectors onto the cut end of the track. Since ScaleTrax is designed more for permanent layouts it looks its best when fastened securely to the tabletop and ballasted.
In terms of the "best" track system to recommend, my advice is to consider the type, size and complexity of your layout, future layout growth, personal interest and desires when it comes to laying tracks, and comfort with soldering jumper wires to eliminate dead spots. The biggest consideration of course is budget - both track systems will cost more than traditional O scale track, so it will be less attractive to switch track systems after a sizable initial investment is made.
Hope this helps, and good luck!
-John