The 830 has faster read/write speeds than the M4 so it will be faster in the main HDD slot. People say the difference between the two is not noticeable. I tend to disagree though. Maybe it's just a mind thing on my part, but just as people tend to want the latest and the greatest computer, I feel the same about my SSD.
I think your Apple SSD is a SATA II so you can put it in the optibay. I'm currently running a Samsung 470, which is SATA II, in the HDD slot. I plan to buy a Samsung 830 when I find one at the price I like. The 830 will go in the HDD slot and the 470 will go in the optibay.
As you can tell I am biased towards the Samsung drives. Have had no problems with the 470 and have yet to require a firmware update after almost a year in use. If I'm not mistaken the new MBP's are using Samsung drives/controllers.
Check out the Samsung site regarding the firmware update process. It's not a complicated deal from what I read. I know a couple of folks that I trust with Windows machines, so I'll cross that bridge if and when I come to it.
Don't get me wrong the 830 and the M4 are both good drives. I just personally feel, based on my own experience with the 470 and the research I've done, that the Samsung drives are a tad bit better.
There are a couple of good threads here on MR regarding the 830 hand the M4. Do a quick search and read what other members have to say. That might help with your decision.
----------
There will be a speed difference with having the new drive in your main bay, however, it is debatable whether you will perceive a difference in speed.
Either way, it sounds like you have very little choice since any drive you buy now that is SATA III will have to be put in the main bay (not the optibay).
On another topic. Noticed you're running 1600 MHz RAM. Any noticeable difference in performance? There's a guy on this forum trying to sell some and I'm debating on buying from him.