Difference between "Track Frame set" and a "TimeTrial/Triathalon frame set" I am going to make a custom fixie bike (possibly with a front brake) that I will use to race on tracks and even commute with.  

I have been looking at a Planet X frame, and they seem to have very high quality frames for decent prices.  But I am not sure if I should get the track frame or the time trial frame.  What are the differences between these style of frames?  The time trial frame is $150 more than the track frame, is it actually that much better?

I am going to put time trial handlebars on the bike.  I also plan to use a belt instead of a chain to cut down on weight and maintenance.
You seem like you want three different bikes all smashed up into one, and the result is not going to be good.

First, the time trial frame is not intended to be a track frame. If you're going to race at the track, use a frame and components that are designed for the track. For starters, the time trial frame will have a lower bottom bracket, dramatically increasing your likelihood of a pedal strike, which can be disastrous.

Second, bikes at the velodrome are not allowed to have brakes. Period. You will also likely not even be able to mount brakes on the Planet X Track frame.

Third, a belt drive won't really improve the reliability of a track bike. Track bikes use significantly thicker chains due to not having to fit between tightly-packed cogs in a rear cassette. Having raced a few years at velodromes, I have yet to see anyone snap a chain.

Fourth, you probably won't even be able to install a belt drive. Belts can't be disconnected/reconnected like chains can, so you will have no way to route the belt through the frame to mount it around the chainring and cog.

Fifth, aero bars are explicitly disallowed from any mass-start events at the velodrome. Unless you only plan on riding pursuits, you will need standard drop handlebars.

My advice? Save the creativity for your second bike. The fact that you have to ask this question tells me it's probably your first build, and you're very likely to end up with a frankenbike that's not actually what you were hoping, not race legal, and likely not even possible to build given the requirements / parts you've named.