The term "inland" and "outland" causes great confusion. While there are nuances to the choice (you can get an open work permit once inland sponsorship has been approved, but you cannot do so with outland sponsorship) for most people the issue is one of "which one is fastest". Note that if you do file outland, there is the risk that if they call you for an interview (unlikely, but possible) you will have to travel to London for that interview.
To figure this out, go look at the current wait time. There are two times to consider: the time to process the sponsorship application (CPC-V for inland and CPC-M for outland) and then the time to process the PR application (inland means it is done inside Canada at one of various offices, outland means it is done outside Canada at an Embassy, Consulate or High Commission.)
For example, the current time for CPC-V (inland) is approximately 11 months for the sponsorship application. Inland applications then require 8 months to complete 80%. So estimated total time is 19 months.
The current time for CPC-M (outland) is 54 days for the sponsorship application (so let's call it 2 months). CHC London says they complete 80% of applications within 7 months. So estimated total time is 9 months. (I believe that if you have long-term status in Canada you can also choose to process through Buffalo, but Buffalo lists an 80% time of 11 months, so that's not worth considering here.)
Note: this is from the "Check Application Processing Times" link on the CIC website.
Bottom line is you can pick the one that is better for you.