Mid-2014 vs. Mid-2015 MacBook Pro SSD real-life performance question

[Crunch]

OK, I'm trying to decide between getting a Mid-14 vs. Mid-15 rMBP. I understand the SSD in the 2014 model is a PCIe 2.0 x2 SSD whereas the 2015 model has a PCIe 3.0 x4 one.

I won't be doing much in terms of sequential transfers, but doesn't the faster SSD also have significantly higher IOPS? Doesn't everything from opening applications and moving around between applications run faster?

I would LOVE to hear from those of you who have had experience with both types of machines and to figure out what to do in the next few days.

Available to me for about the same price, I can get either a (Mid-2014) 2.5GHz model with the NVIDIA 750M (3-year old tech at this point) dedicated gfx and a 512GB SSD but it's, of course, the slower SSD

-OR-

a (Mid-2015) model with 2.2GHz and NO graphics chip other than the Intel Iris Pro 5200 and a 256GB capacity SSD, but this is the faster SSD.

So that's 300MHz on the CPU plus the NVIDIA 750M w/ 2GB GDDR5 AND 512GB capacity vs. 256GB, but it's the fast SSD.

I love FAST SSD's so much that this decision is such a big deal. At the same time, maybe this is now overkill?

Thanks a ton!