KT150 to KT120..or KT120 to KT150

@daveyf 
Good to know.  I have some friends with CJ older amps that swear the KT120 sounds better in THEIR particular amps, i.e. not designed to run KT150s.  I believe what your designer friend is saying maybe for that amp circuit. Fast forward, now CJ has amps designed for KT150s, 2x the cost, go figure.  Optimized to run KT150s.  All new amps, new circuits... 

In reverse, my current mono tube amps are designed to run KT150s natively, but I thought I'd be smart and try KT120s at first hoping they'd sound more like EL34s did in my old amps.  Amp designer said I could try KT120s and see.  Nope, no-go. They sounded more dry, less musical, kinda plain, some call it analytical, less musical - and lost some air up top. Just not engaging in these amps.   Yanked the KT120s and replaced with matched pairs of KT150s, and wallah (for these amps). More top air, more bottom, and a mild mannered midrange.  So, I then ended up back to rotating the former pairs of vintage input/drivers in my amps to bring out more midrange, it was better. Then, went to better coupling caps, and even better midrange with KT150s, now I was "there".  When I tried these same changes with KT120s, just did not help all that much. With KT150s, worked really well.  

And, opposite in my buddies integrated amp that was originally designed and voiced for KT88s.  The new KT150s were not cutting it in HIS amps.  Less musical. Yanked out KT150s after 6 months, then went with PSVANE KT88s, and wow they worked so much better in HIS amp.  

Finding the right tube(s) inpu/driver/output for the circuit seems to be the ticket for me.  I don't know if I believe in the whole "universal" amp theory and just changing the driver.. may work with some combos, not all.  My designer knew KT150s worked better in my amps circuit.  

What amuses me a little with the amp designer in question, is that not only does he preach that the KT150 is worse than the KT120, with no proviso that it could be in his circuit, but not others; but also the fact that he did not believe in tube rolling or NOS tubes until recently and was known for telling everyone he was in contact with about this. ( wherein his new marketing manager must have told him that this could be another profit source, and apparently magically changed his mind!). We really do have a lot of 'interesting' fellows in this hobby on the manufacturing side, IMHO.

I used both in my previous amp, a push pull 85wpc, and my current amp, a single ended SEP Class A...in both cases I also preferred the 120s as they seem relatively clean and clear sounding, where the 150s just sound forced somehow. Now I use neither as Gold Lion KT77s and 88s and Tung Sol 7581s all sound better in the SEP amp than 120s or 150s...currently running an old unused pair of Sovtek KT88s...they're great!

@wolf_garcia
On that note, KT150s were not particularly special in my former SEP Class A DH Inspire Hot Rod amp either. Seems most like the smaller tubes with the smaller transformers in that amp. I concluded that KT150 tubes were a waste of $ and no value-add in those smaller footprint amps.

However, KT150s in my current Quicksilver Mono 120 tube amps is at an entirely different level of sound, stage, and enjoyment for sure. Transformers are 2.5x the size, higher plate voltage used, and yet I can manually bias them down to about 40ma per tube, and it sounds closer to something between EL34 and KT88s in triode mode in my prior Cary or Inspire amps. Owners stating something similar about the higher $ CJ amps specifically designed for KT150s.

Seems the specific amp design, the circuit, and use of sizable transformers sure make a difference for running KT150 tubes in their most optimum state. Now seeing more amps designed just for KT150s.  

As an owner of an amp designed for the KT150, the ARC Ref 150 SE, my only comment would be why even wade into the weeds as to which is better is any particular amp when New Sensor has terrible QC in general and most terribly with the KT150? For over two years now, New Sensor has failed to make any improvements in their QC while continuing to put out KT150's to the peril of the consumer. 
If you buy a quartet or octet of "matched" KT150's from virtually any source, you are doing so at your own risk. And in case I am not clear to you, I am saying that most vendors who claim to match tubes do not do so very reliably. They either lack the necessary equipment, the necessary know-how, or both. 
If your amp has fail safe features such that an arcing tube will not necessitate any repairs and your supplier is willing to give you 60 days, I could see venturing into the 150 waters. Otherwise, I would stay away unless you have money to burn AND don't mind the inconvenience of taking your amp in for repairs/sending it back to the manufacturer for repairs.