I have never read the Sealed Nectar, it is a Twentieth Century Sirah, not a Classical work. But it was awarded some Sirah prize by the Saudi government for the best English language Sirah, also Islamic apologists always recommend it to people who they believe do not know much about early Islam. When I debate with Muslims they often recommend this book to me, they assume that I have not read anything, and it is obvious that they themselves have never read Ibn Ishaq because if they had then they would at least know that I am not a novice on the topic.
Why would someone recommend the Sealed Nectar to someone that is quoting Ibn Ishaq in a debate? The information in the Sealed Nectar comes from Ibn Ishaq!
I would recommend you read this:http://www.amazon.com/The-Life-Muhammad-I-Ishaq/dp/0196360331/
It is the English translation of Ibn Ishaq by Guillaume.
The actual work of Ibn Ishaq is lost, the surviving version we have is from Ibn Hisham, who copied it but removed the parts that he considered might be "offensive to Muslims". So for example the part about the torture of Saffiya's husband and the brutal method of executing Umm Qirfa, Ibn Hisham took these out. The only reason we know about these incidents today is because Al-Tabari recorded them in his history writings and quoted Ibn Ishaq.
So what Guillaume did is translate Ibn Hisham but also put back in the parts from Al-Tabari that we know Ibn Hisham removed. He lets the reader know which parts of the English translation are from Ibn Hisham and which parts are from Tabari by putting a little "T" in the margin wherever he incorporated the parts from Tabari. By doing this he has partially restored a lost work, of course there may be other parts of Ibn Ishaq that were removed by Ibn Hisham and Al-Tabari did not quote them, and these are lost forever.
So anyway I recommend Ibn Ishaq and not the Sealed Nectar, I have not read the Sealed Nectar but I think it is essentially a dumbed down and sugar coated version of Ibn Ishaq.