MX100:
Pros: MLC NAND. Proven and reliable
Well engineered: with those extra capacitors for power protection it beats most other consumer grade drives. It will allow for a final write if there's ever a power failure which saves your data. That's enterprise grade stuff.
Value: absolute killer price point.
Build quality: Micron is a fab. They designed it, made it and sell it. Should be superior to all those 3rd party vendors.
Cons: middling performer.
Watch your sizes. The 128GB isn't the same as the 256 or 512. And only the 512 is referenced for the speed specs.
840 EVO:
Pros: Well engineered. Samsung has really set this thing up.
Sizing: One of the largest size ranges with a 1TB available.
Build quality: Samsung is a fab... yadda, yadda, yadda.
Speed: they are among the best on the bench. Problem is that in the real world it's not as noticable. Blindingly fast, really fast, very fast or just fast. Can you tell the difference?
Software: their Magic software is the best by far for a software suite. Very useful utilities to optimise your system. Most drives don't come with *any* software.
Cons: Value: Surprisingly, they've lost this battle to Micron. If they can match pricing then this goes away.
TLC NAND: It is an issue though it looks like the engineering is holding up. Still, it's something that with all the chips out there, Samsung is the only one to go this route. The fact that they are rebranding TLC as something else is telling.
TL:DR Follow the money. Buy the cheapest you can get.