MyTraffic uses entirely random schedules, but allocates aircraft by region. If you want masses of aircraft in the sky, this is by far the best option (although I still think that "from a distance the aircraft look reasonable" is a good description of many of the paints). It works in DX10 by not using the FS9 models (which are reducing in number with every release anyway) but is unsuitable for those wanting realisim - although it is very powerful in terms of editing tools, if you have the patience to do so.
JustFlight Traffic X uses ancient timetables. Many of the airlines in the default install ceased operations, merged into larger companies or rebranded several years ago. It is, however, the only
pure FSX traffic out there. It is also editable, also has period traffic (not back to the 1940s unfortunately... I'd pay extra for that!

) but isn't as well supported as MTX.
Flight1 Ultimate Traffic is not FSX native in any way. There's supposedly an FSX native version coming, but no news of when. According to the F1 website, "[FONT=Arial, Helvetica, sans-serif]
Important Notice - This product has been discontinued and is not available for purchase any more due to the new version (Ultimate Traffic 2) coming out soon." -[/FONT] that statement, however, has been there for months.
There is
no FSX native freeware traffic out there. Any time you download a package from the big freeware groups that claims to be FSX AI, it is FSX format flightplans, using FS9 format models. However, this is the only option for current, real world, flight plans.
It's swings and roundabouts, depending on what you want. Having tried all but Ultimate Traffic, my choice is TrafficX, but for someone less picky than me about what they see around them, MyTrafficX is by far the best in terms of coverage, variety and has excellent support.
EDIT:
The best thing is going to your local International airport and seeing all the real companies that your use to seeing there.
That's one of the two very big problems I had with MTX: The two airports I use most frequently - Birmingham and Belfast Internationals, are absolutely heaving with British Airways jets. You will not see a single BA tail at either in the real world. Belfast is a hub for BMI Baby (not a separate airline in MTX), Aer Lingus and by far the largest operator is Easyjet. In MTX, you see very little but Ryanair and British Airways. All BAW flights out of Birmingham were transferred to FlyBe several years ago and they and Ryanair are by far the largest operators at Birmingham, yet there were no FlyBe, only rare BMI Baby and Ryanair flights in amongst the great morass of BAW tails at Birmingham in MTX.
If you go to a Royal Navy Helicopter base in MTX, it is crawling with European F-16s, USAF freighters, Scandanavian ASW aircraft... This is the way it is designed to work. It is not designed to be accurate, it is designed to fill the world with traffic. For most people that's not a problem, but I very quickly uninstalled it because it's not what I want.