A good answer already given; I am taking a slightly different approach. It seems to me that you have two concerns, first the efficiency of the power supplies, and second RFI.
So lets take efficiency in terms of $.
Assuming maximum use 24h/day, 1 year. Taking 10.42 cents per kWh (source). And lets take a "loss" of 5.4 Watts from your details; 9 Watt in, 3.6 Watt out.
Lets see what this is in a year:
0.0054 (kW) * 24 (h) * 365 (d) * 0.1042 = $ 4.93
Lets take a theoretical switch mode power supply with an efficiency of 90%, and output of 3.6 Watts, so the "loss" is 0.4 Watts, and calculate the same for a year:
0.0004 (kW) * 24 (h) * 365 (d) * 0.1042 = $ 0.37
As expected it is a big difference in annual cost: about four and a half dollars. However, the more efficient switch mode power supply have to be purchased, and probably/maybe EMI/RFI filtering is needed to clean up interference.
So now the approach of this answer is an economical one:
Taking into account purchase price for the replacement, possible purchase price for EMI/RFI-filters, and the difference of the running cost. What is it worth to you; running a low-RFI high-loss power supply, versus, a possibly-high-RFI low-loss power supply ?
And I purposely omitted "durability" in this answer. In my opinion linear power supplies have a longer life than switch mode power supplies. However I have no data to back up that claim. If that is true then in the economic answer you will have to figure in depreciation of both, and set this off over time/annual.
In my experience over the years, the lower cost switch mode power supplies are prone to failure and high RFI. In regards to RFI: I have had to put filter on both input as well as output of those cheap switch mode power supplies. However I have some more expensive ones, which are OK.
One experience I had was with mobile/cell phone chargers. Two exactly the same branded phones (which shall not be named, but which is in the news lately due to burning phones), with the original power supplies, both PSU exactly the same, same print on the casing, same type number and other indicators/writing... except one showed "made in China" the other showed "made in Taiwan"... The one "made in China" had no noticeable RFI (I did not do measurements, other then SDR-waterfall observations), the PSU which was "made in Taiwan" had RFI all over the place, to a point that when the phone was charging I could not operate on HF. So even when you purchase known-good-brand switch mode power supplies, you can get a bad one. The obligatory YMMV applies here.