Honestly, I doubt you'd notice a difference. I believe the Samsung 840 is faster than the crucial m4, but not in a way you would notice unless you time exactly how long it takes to load a map or open a program everytime (and we're talking like 2 second loading difference in games and negligible in programs like Word).
When you talk about noticeable differences it really doesn't apply SSD vs. SSD, because your really not going to notice the difference. Now HDD vs SSD, sure you WILL notice a speed up. IOPS is the number of input output requests the controller for the SSD can handle per second, and is more a benchmark test that has nothing to do with real-world performance, and you shouldn't concern yourself with IOPS numbers on SSD's.
Pretend I had a Crucial M4 128gb SSD. Would the performance be NOTICEABLE if I switched from the Crucial to a Samsung 840 120gb SSD?
Short answer: No.
Ignore synthetic benchmarks. They use specialized synthetic programs to drive the SSD as fast as it can go. There will be up to 32 outstanding operations at a time.
That is perhaps applicable to a server where high IOPS(Instruction Operations Per Second) might be a valid metric.
As a desktop user, you will be doing things one or two at a time. There, the response time of the ssd is the most important metric. Unfortunately, all modern SSD's have about the same response time, so that does not sell.
I don't know why you might switch from Crucial to Samsung.
Certainly not for any meaningful difference in performance.
Thanks so far, how is Samsung more reliable than Crucial? It the build quality or failure rate?
The only numbers I see might be considered satisfaction rate.
And... they may not be all that reliable.
Aggregating all crucial or intel/samsung drives into one generalization is not good.
Over time, products get better, and we often see poor results from cheaper early products that are not representative of later updated products.
At one time, I looked at the most popular 120gb SSD's on newegg.
The percent with 1-2 eggs(not good) was in the 6% range for Intel and Samsung. It was twice that for other brands.
Not a great method, I know. But, if you look at a candidate ssd with some sort of history, you can get some idea of reliability.
Another way would be to access the ssd support web site, and see what sort of issues are prevalent and what the resolutions are.
In the end, it does not matter much so long as you buy from a place with good RMA and customer servicie policies.