@TomSpliker's great answer mentions that ESA might be looking at producing Radioisotope (powered) Thermoelectric Generators (RTGs) based on the radioisotope americium-241. 241Am is a "kinder and gentler" isotope than 238Pu to access since it's sourced from reprocessing nuclear waste from power generation and doesn't have to be "made to order" in a special reactor.
But it decays five times more slowly, with a half life of ~432 years versus ~88 years for 238Pu, and the alpha decay is accompanied by some emission of X-rays. The additional radioactivity of the decay products which increases over time for the two are different as well.
Suppose I had a spacecraft or rover that needed 100W of electrical power, and due to uncertainties I needed to design the mission to allow for either 238Pu or 241Am based RTGs, I'd have to compare all aspects, including mass, additional radiation of the decay products, mission life (say EOM is 80% of initial power) and size.
How would the two then compare?