This is direct from nzxt website, with a 120mm AIO mounted at rear. Not sure how you figure its "too close to the cpu" unless you didn't first remove the fan mounted to the case. Same with top mount.
If a pump is working correctly and you have good flow, the temp difference in the tubes is generally only 1-3°C different. Not enough for ppl to tell any difference, both should feel warm after acclimating to whatever load.
120mm are not really an issue. They generally perform almost exactly the same as a CM Hyper212 Evo, and thats the gripe, they cost double (at the time) what that air cooler does, for the same performance, same capacity, @ 140w. Nowadays, the Hyper212 is @ $50 and an Aio @ $60 and Noctua cost more than that.
A decent aio will last @ 5-7 years, depending on use. Most ppl dump their pc about the same time, sometimes earlier. The new pc, they'll get a new cooler. So longetivity is generally a moot point. A NH-D15 I'd prolly keep, a CM hyper212 that's 7 years old? Prolly not since the fan would be on its last legs anyway.