Ease vs. cost and the size of your contract. IF you are looking for a small add on, then going through Disney will might be easier as small contracts, even at SSR, are not as easy to find, especially if you want to have the same UY. Buying through Disney always gives you the same UY.
However, if you are in no rush and want at least 100 points, you will save a lot of money (around $30.00 per point) going resale. But, you are looking at a 2 month period, assuming you pass ROFR the first time. If not, it could take 3 - 4 months from start to finish.
Good luck!