You'd be hard-pressed to find anyone who wouldn't agree, as far as performance, between the two goes.
It's either old timers, Collins collectors or people like myself who enjoy the nostalgia factory. The whole I love TUBES, ANALOG METERS and KNOBS thing. I'd take the SDR (Flex) any day overy the "antiquated" Collins equipment. One could always choose to use a tube leenya (lol) to scratch their nostalgia tubes, and knobs itch.
As a matter of fact I wouldn't be surprised to find many Flex (SDR) owners driving Heathkit, Henry, Collins ect ect lennya's along with their most modern technologically advanced SDR equipment. The cost of modern "superior" amplifiers is outrageous compared, to the name brand amps I mentioned above. With a few mods those old amps could easily be driven with modern transcivers (SDR included.) Those old smps still hold up when compared to something costing 4x (and more) the price of a brand new LDMOS amp. Sorry for being verbose. I "on occasion" go off on tangents. 73