Use
Anandtech's Bench to compare the SSDs you are considering.
The micro-controllers used in SSDs can access a finite number of "channels" in parallel (usually eight or ten IIRC). The size/capacity of the flash (memory) chips used in the different capacity SSDs is usually the same; simply fewer are used in smaller drives. A 128GB drive will have half the number of memory chips that a 256GB drive has, attached to half as many of the controller's channels, achieving half as many operations in parallel, therefore achieving half the speed. This is massively oversimplified (and a bit wrong) but it gives you that idea why bigger SSDs are generally faster than smaller ones.
There are exceptions to this. After 256GB (with the current NAND chip densities) there is generally no performance increase with going to larger drives. Strangely, Anandtech's Bench reports that the 512GB Samsung 830 is fractionally slower than the 256GB version.
Intel, Samsung and Crucial are all very well regarded SSD manufacturers. Intel's rigorous validation (testing) generally puts them at the front of the pack for stability and reliability, though Samsung have made a name for themselves with extremely reliable (and fast) SSDs. Crucial is not far behind the other two (and usually a little cheaper).
At the moment 256GB is the "sweet-spot" of price/performance/capacity. The one thing to be cautious of with Samsung SSDs is that their firmware can only be updated under Windows; if this is an issue for you, then you're best sticking with Intel or Crucial.
...also; commas, apostrophes, capital-letters, and the word "you" (rather than "u") are all beneficial in not appearing to be a moron (which I don't believe you to be*)
🙂
*N.B I am a patronising bastard, so it's ok for me to appear as such.