SIP is better for a job that only requires moderate paint correction. It's an awesome polish, especially via rotary.
M105 is better at removing heavy defects- deep swirls, scratches, etching,
water spots, etc
Both actually probably finish out about the same, so if you had to buy just 1 M105 would "do more," but at the same time it is not a good idea to use overly aggressive products when possible.
Meguiar's preaches (as do others) to use the least aggressive option that effectively fixes the problem- in some cases, SIP would be the answer.
All that said, I have both products, and when do full corrections (90% of my work is this) I reach for M105 over 90% of the time after testing.