The gas system often gets little thought from people until they build their own rifle. While I am sure he is out there, I have yet to hear of a guy who changed the gas block on his rifle just because. Generally it is only done to allow a new rail that would not fit over the old gas block, or to add an adjustable gas block.
Aside from VLTOR's cool side cuts,and the bits I missed with photoshop on the spikes, there ain't much difference between them.
In terms of gas tube, there are not many options. Try not to pay much over $10 for one. You can get any brand, but chances are they are just selling the same tube as DPMS. If you need the blackest of black rifles, you can get a melonited tube from Spikes.But they have not been as available lately and they are no longer on Spikes' website, so you may have to act fast.
There are quite a lot of gas blocks available. My favorite is the standard low profile block. I don't know who originally came up with the Noveske/VLTOR/ Troy/Spikes YHM (and several other brands) gas block style but you should get it from Spikes, YHM or Troy. They run half to a 3rd the price of the others. There are some differences, but they are subtle and none of the 3 I mentioned to get have performance issues. Paying more for a logo is cool, but not if you can't see it without a flashlight and a good angle.
BTE Railed adjustable gas block
There are again quite a few options and many of them are different. Personally, I try not to attach anything to the gas block though. It gets very hot and I don't want to torque it. However some people love it for sling attachments and many people love it for a front sight mount. Sadly, due to my strong aversion to mounting anything to the gas block, I am not well schooled in a good choice for a railed one. JP Rifles and Battle Tested Equipment both make solid looking options though, as do a couple other brands.
Being able to adjust the gas block allows you to tune your rifle, not just to the ammo you are shooting, but also to the return spring, buffer and BCG mass. It also allows you to maximize your muzzle velocity. By how much of a difference? Right now I have no clue.
But eventually I will post data a buddy and I are in the slow process of collecting on that subject. Probably the most tested and used adjustable gas block is made by JP Rifles. They make both railed and low pro adjustable. How low pro? Not very. It won't fit under some of the smaller rails like Troy's TRX extreme. They have a new one out (the one in the link) I have yet to see that may fit though I doubt it. It does, of course, fit under the JP Rail though.
BTE micro adjustable gas block note the solid set screw
PRI, Spikes, BTE and Syrac Ordinance all make a true micro/low pro adjustable gas block. I can't personally attest to any of their performance or reliability as I have only heard second hand that they are good. For function alone, I would take the PRI or BTE over the Spikes/Syrac though, due to the larger set screw.
There is some fear and rumor that adjustable gas blocks work loose over time and leave you with a single shot rifle. I own 2 JP adjustable (older ones) and I maybe know of 20 other guys that run them (almost all JP). While I have read the stories on the internet, between me and the other guys, we have maybe fired 80,000 to 100,000 rounds between us and zero reports of problems. It is true that the heat greatly reduces the effectiveness of loctite. But all the same, I trust the gas block.