As the saying goes, "It all depends....".

Placing SQL server  on the same server as SPS will eliminate any network latency introduced by splitting SPS and SQL onto separate  servers, however, now you have both SPS and SQL fighting for RAM memory. Neither will get all that they want so you loose performance  due to increased disk I/O and increased CPU demands.

One machine is cheaper than two. (Hardware and OS licensing)

Raid 5 is a cheaper than Raid 10, however Raid 10 is faster in a highly updated environment due to heavy parity disk updates needed in a Raid 5. Raid 10 is faster in a heavy read  environment because it has both mirror copies to use when fetching data. However, Raid 10 really only shines when you have split the mirror copies between separate disk busses. Placing all the drives  on the same disk bus will slow I/O down due to bus contention. You cannot have two drives delivering data simultaneously on one bus. Most servers offer single bus disk cages. A seprate bus usually means getting an additional disk enclosure and cable to connect it to the external (and must be separate) bus connector on the original server Raid controller card.

Again, best performance is higher cost.

Will your application demand top performance in the first month? First year? Can you start smaller and expand later? The best course demands that you know what your real performance needs will be.