For an average customer, there is no noticable performance difference between a 5400rpm and 7200rpm hdd. Ssds are faster by a factor of 1000!
I have to agree entirely with this. Hard drives are hard drives. With the exception of the hybrid (SSD/HDD) models, the end result is still very slow (by SSD standards) seek times. This is what makes an SSD feel 1000x faster than a spinning hard drive. Even a low end SSD will crush a hard drive for OS uses. There is no way that even a 7200rpm hard drive with it's 10ms seek time will feel as responsive as an SSD with <1ms seek time.
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Not entirely true. Yosemite is a memory hog that can use ~4GB at fresh idle. So the base model with only 4GB total ends up doing a lot of memory paging to the slow hard drive. With 8GB on the mid-range, at least it isn't making constant trips to the hard drive when just doing general tasks (as OP describes).
Ultimately both should have an SSD, but you have to spend nearly double the price of the base model to have it configured with one as it isn't available on the base unit.
PCIe SSD doesn't make that much of a difference.
Old school thought process..... Both Windows and OSX automatically loads everything into RAM anymore. "Free" memory is wasted memory.