There are tradeoffs with all the types. O gauge tubular is the cheapest (though switches are not exactly cheap), there is a ton of it out there, but also looks like toy train track. You can do a lot with O tubular, you can add extra ties to it or even chuck the metal ties and glue/spike the track to wood ties...but would take a lot of work.
Atlas makes nice track, but it isn't cheap, it is pretty realistic looking. Sometimes you can find it used as good prices, but it isn't that easy IME to go the used route. Atlas has had trouble with availability in recent years, seems like they are now routinely shipping, but could be an issue in the future.
Likewise MTH scale trax is nice looking, if somewhat fragile from what I have seen of it, but it is expensive and can also be difficult to find.
Gargraves to some isn't realistic enough (oversize ties, rail profile is tubular), but it is cheaper than the other brands, and combined with their own switches or Ross, it seems to be a very popular combination, just from seeing layouts in magazines and online a lot of people go this route. Ross offers a lot of switch combinations in both tinplate style or in more scale (ie the diverging route is angled, not curved) so you likely won't have any trouble finding it (it is made here, so you don't have the shipping drama you can have with stuff made overseas).
Fastrack has its positives, can get it anywhere, not the most realistic of track but has built in roadbed. Not particularly cheap, also somewhat noisy, so that may be a drawback.
Personally, when I get around to actually being able to build a layout, I suspect I'll go with gargraves and ross, but in the end it will come down to a)your sense of esthetics ie which track actually looks better to your eyes and b)budget. Nice part about the hobby is unlike 40 years ago, when your choice was either tubular or gargraves, there is a choice.
