None of Apple's marketing that I have seen advertises the actual speeds of the SSD, so technically Apple is under no obligation to deliver same-speed SSD's, as much as that bites. Engadget found the following in their testing:
Samsung SM128C SSD:
251 MB/s read
214 MB/s write
Toshiba TS128C SSD:
203 MB/s read
184 MB/s write
So the Samsung is 48 MB/s faster for reads and 30 MB/s faster for writes. Considering that a lot of hard drives only run at about 60 MB/s for reads total, that seems fairly significant, but in my experience once you hit 200 MB/s everything else is just gravy (unless you have those new 500 MB/s SSD's, whoohoo!). In real life, I haven't noticed a difference between any of my SSD's above 200 MB/s or so. I do notice a difference going from my older 150/170-ish SSD to the 200+ model, but from say 200 to 250 it's not quite as noticeable. There is a difference, but it's not huge.
Is it worth returning? That's up to you. If you do a ton of stuff that requires speed on the boot drive and it's not an enormous hassle to go to an Apple Store and do a swap, then sure, why not. But most people just surf the web and do word processing, so I'd say meh.