The crossover point depends on how the weight is distributed between the radioactive material, the shielding, and the radiators.
The first two favor plutonium, the last one favors americium.
The first two favor plutonium, the last one favors americium.
It's still not a close call, as far as I can tell (I'm not a domain expert). If you plan for even 20 years operating life, then your initial heat dissipation (for Pu-238) would be 1.17 of the nominal power at t+20 years, due it its radioactive decay. The largest RTG, GPHS-RTG, has 13.0 kg of "housing and fins" [0]; if you scale that linearly by 1.17 that would add +2.2 kg.
On the flip side, the isotope mass that emits 4,500 Watts of heat goes from 8.3 kg of Pu-238, to 39.5 kg of Am-241—that's +31.2 kg.
This is relative to a total mass (GPHS-RTG) of 56.0 kg.
(To preempt anyone complaining that this are small quibbles: the total dry mass of New Horzions [1], which contained one of these RTG's, is 401 kg).
[0, pdf] https://ntrs.nasa.gov/api/citations/20080003866/downloads/20... (table 1 on page 3, "State-of-the-art RPS. Comparing existing GPHS-RTG with near-term MMRTG and SRG110")