I don't have a million bucks to spend on my layout so I always have to make compromises. For switches, I use Atlas Customline #4 and #6's for yards and industrial spurs that don't come off the mainline. I'll even use Atlas Snap Switches if I'm really in a bind for space. For passing tracks off the mainline, wyes, and crossovers, I use Walthers Shinohara # 5 and #6 turnouts and would use #8's if I have the room. They are a good value for the price and look more realistic than the Atlas products. Both brands are generally very reliable although Atlas switches commonly need to have the points filed to get a sharper angle of departure. You also need to check the gauge through the frog as it is sometimes too narrow or too deep. This can be fixed but it needs to be done to get reliable operation. Shinohara turnouts usually require less prep work to get running reliably. Rex has pointed out the issues with both brands. I think Peco is way overpriced for what you get but they are very good looking switches. The most important thing with any switch is how they and the surrounding trackwork is laid. If everything is level, in gauge, and the points are sharp enough and move freely, your trains will run well. Scrimp on these and you'll have problems regardless of the type of switch.
Code 100 vs Code 83 is strictly a matter of preference and price, Code 100 is less expensive but it also is more out of scale than the Code 83. Code 83 is closer to mainline rail height and the ties are generally finer scale but it's also more expensive. Both types of track will work equally well as long as they are laid correctly. I can't overemphasize that your care in track laying is way more important than any brand of track or switch.
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