You might get some disagreement that the Paragon sound is inferior to the Tsunami. I would rate it equally good, personally, if the Hybrid series are anything to go by, of which I have two different production examples, the Union Pacific 2-10-2 and the Pennsy Q2. However, it is always subjective, so if the Tsunami is your choice, and you can get a DCC-ready model, then by all means....
You might want to investigate the new QSI Titan series. I recently had two of them installed in two different steamers, a N&W A Class 2-6-6-4 and a Heavy Mountain from Bachmann Spectrum. I think they are wonderful and have excellent clear sounds.
To directly address your question, I think you will like either K4s, but you might not be aware that the first run, in the Platinum Series, had a QSI of a much earlier version. During that time, QSI, Lionel, and MTH were before the courts. QSI elected to disable their BEMF low-speed control at the time to protect themselves from any further liability. All Paragon Series locos were affected at that time, with the K4s being among them. I upgraded the main chip in the decoder about 14 months later when QSI and MTH had settled and QSI were able to offer improved sounds and restored BEMF. It made a world of difference in my opinion. So, you will get a limited version of the first QSI's if you elect to purchase one of the first runs of this Pacific class locomotives from BLI.