Both nandrolone deconate and nandrolone phenylprop are equally supressive. They are the same chemical, nandrolone, just esterfied differently to give different half lives. The advantage of a longer acting ester is that a more stable blood concentration can be acheived, but the disadvantage is that the drug persists for much longer in the body. The deconate ester presents a more difficult recovery because the drug takes longer to metabolize and get clear of the system. However, your HPTA is going to be in the same same condition after 8 weeks of NPP as it would after 8 weeks of Deca. You would just need to wait longer for the deca to clear the system and begin recovery (and while you are waiting for the drug to clear your system, it is continuing to further suppression).
(this is where my knowledge gets a bit shakier)
The degree of suppression that a drug causes depends on its level of binding to the androgen receptor. This is why primobolan which has a low affinity for the AR is considered a minimally suppressive drug. The higher the affinity for the AR, the more suppressive the drug is.
Nandrolone has a higher affinity for the AR than does Boldenone (eq). Thus EQ is less suppressive than Deca, although its half life is actually longer. Trenbolone is even more suppressive than either of them, and as an acetate ester its half life is way shorter than Deca’s or EQ’s. The longer half life merely means the drug will be active for a longer period of time, complicating recovery.
There are other contributing factors, but this is the broad overview.