Is there one that provides a better variety of vehicles over the other? Also, can any of these increase the volume of vehicles?

I cant really answer your quiestion but if you want more vehicles go to your settings and change "vehicle density" to a higher level.

I use Traffic Load. It follows the same idea as Traffic Flow, but uses the entire list of civvie cars instead of a set 20, and always spawns all Taxis and the Bus. It can be set up with custom densities (not really sure of the details, though) and custom car models to add.

I cant really answer your quiestion but if you want more vehicles go to your settings and change "vehicle density" to a higher level.

What file can I find that in?

Simply copy over TrafficFlow.net.dll and TrafficFlow.ini to your /scripts folder in your GTA IV or EFLC directory.

What file can I find that in?

You dont need to dowload anything...just start your game them pres Esc go along to graphics then down to vehicle density

I went with TrafficLoad, I really like it. Thanks guys.

TrafficLoad mod: www.gta4-mods.com/script/trafficload-f9780.

As The Loot pointed out, its a lot better than TrafficFlow. Just make sure if you try it to set "CustomDensity=1" and not 0 which is how it comes by default.