How do I pick a good gas tube? Is there much difference between brands? I was looking at getting one off Midwest industries site with a few other things.

Not much difference other than some being stainless and some being nitride.  No difference in performance.

I don't put much thought into a gas tube. As a matter of fact I put so little thought into it that I succesfully ordered the wrong one 3 times in a row plus one more for a friend's build that I ordered then ordered a cheaper one but I was to late to cancel. Now I have 2 spare mids a pistol and a rifle length.

Might start my own business selling tubes...

Gas tubes are essentially stainless steel brake tubing with some special bends and finishing.  There isn't a lot to go wrong with them, and there isn't a lot of difference between tubes.

With that said, go with a good vendor.  MI is great people and they stand behind their products.  As long as you buy from quality vendors, you can shop for the lowest price for the tube you want.

While there is no "need" for a gas tube to have a nitride/Melonite/QPQ/whatever treatment, it does tone down the bright stainless tube.  Don't go with an expensive tube just because it's got a gee-whiz treatment, or because the maker says they formed it on the thighs of young virgins, or any other snake oil stuff.

get it from a host vender on this sight and you should be good

This one is made from inconel and pretty much guaranteed not to fail even after emptying 5-10 Betas on full auto.

http://www.vsevenweaponsystems.com/products/v7-extreme-environment-gas-tube

I used Seekins Precision gas tubes for 2 builds.

http://www.seekinsprecision.com/melonited-gas-tube.html

I've used Spikes and will again. The nitride or melonited tube is black and stands out less under the handguards.

I wonder why the v7 gas tube has to be installed in the gas block before mounting the block.

A lot of people do this; it makes it easier to a) get the tube pinned in the block and b) get the tube aligned through the barrel nut (the gas block gives you a "handle" to align the tube) and c) the tube helps you align the gas block to the barrel's TDC.

I've always installed the tube into the block before mounting the block.. Really I've never heard of it being done the other way. Am I missing something?

Nope.  It's just a "style" of assembly.

Some people get the block aligned on the barrel, then install the barrel, then install the gas tube.  In some cases it's easier.  For example, my 8" 300 Blackout barrel would have been a royal PITA to install the block on if I'd waited until the barrel was installed, because the block fit VERY tightly on the gas port journal.  There was no way I could have done any aligning of the block with the barrel installed in an upper.