You basically summed up what I thought. The IBM has native driver support in windows 7 or Windows Home Server- You just slap it into the slot and turn the machine on. Done. No driver needed. That's always a good thing
The IBM plays very nice with Esxi and pass through for Virtual machines / HDD's and I believe there is some issue with the SASLP. ( I also believe you might need to flash SASLP for support of 4TB and 3TB HDDs ??) IBM has native support of drives larger than 2TB but requires flashing to IT mode for invisible HBA function.
Lastly, You are correct in the x4 versus x8 speed. I do run 8 full speed 7200rpm Seagate 3TB HDD's on my IBM M1015 and run parity validate, check, and updates constantly. The max read or write of my HDD is about 200MB/sec which is 200 x 8 = 1600MB/sec at the maximum.
PCI-E v1.x on the other hand is able to transfer 250 MB/s (2.5 GT/s) per lane so a PCI-E x4 card will be able to transfer of the order of 1000MB/s. Higher versions of the PCI-E standard can transfer more, as an example V2.0 can transfer 500MB/s per lane.
If you were running SATAII drives or SSD's you'd clearly want the x8 speed card. But if your running slower HDD (like older slower 5400RPM Green drives) it's not going to matter much.
Personally, I would only get the SASLP2 if I was buying today (x8 speed). The advantage of that card is it is a native HBA card that will just work without flashing it. The IBM might be better or more compatible but you need to flash it to IT mode. It's not hard to do (I did mine and instructions in my server thread) but it's a negative of the IBM if your keeping score. Many rookies might not want to mess around with that. It requires typing a few commands into a DOS prompt
I actually purchased the SASLP2 from two different places but it was OOS. It was my first choice per ReneTHX recommendation. I then ended up cancelling my order @$145 and purchased the IBM on ebay with bracket for $79 because I did not want to wait for it to come back in stock. I'm glad I did.