You get 1.0x (or normal EXP/SP) for no links, 1.5x the SP for 1 link, 2.0x for 2 links, and 2.5x for 3 links.
There is a cap at 3 links. You will receive no additional bonus past 2.5x normal EXP/SP if you do 3+ links.
Guardian's Favor gives you 1.5x the [ Final SP ] at the end of all other calculations. It only increases the SP by 1.5x, and will not affect the amount of EXP gained.
It's best to link when the link increases SP. When you have an extra, say you have 5 when only 4 is helping, I think it comes down to what leve it is. For example, the group I run with will sometimes go with a 0 link Ranine Reveries after a full 3 link one because the SP is still worth it. If it's not a strong SP leve, then I'd say just blow them all in one shot and better spend your time elsewhere. Edited, Apr 19th 2011 2:21am by Coyohma