Look up the data on all the common rectifier tubes. Find the ones that can handle your amps current draw. The ones that drop the most voltage will be the ones with the most sag.
In considering this feature in your new build, you only need to ask yourself, 'While I was playing the amp, which rectifier did I most often use?'. If you were happy with the diode rectifier, thats it. Sometimes you have to stop scrutinizing an amp and just play through it. Thats the only way to really evaluate it.
FWIW I recently did a survey at some local music stores to weed out unwanted features. The only feature that consistently ranked "NO" was a switchable tube / diode rectifier.
Also, You don't need a tube to create sag. A big fat resistor will do just fine. You could use a tool like Duncan PSU to determine what value resistors simulate the rectifier tubes you want to emulate. There are some subtle differences between the tube and a resistor. But as you've already noted, the difference can be subtle anyway. I bet you could put the tube rectifier and a resistor on a switch and you'd have to toggle it back and forth many times to tell a difference. And then you'll still wonder. So why trouble with the tube. It means you need an extra socket, a 5 volt winding on the PT and of course the tube itself. As well as the occasional cost of replacing it. JM2C. On the flip side a tube does have a certain purist appeal and I do sometimes build an amp with a tube rectifier. And I think deep down I like it better for strictly aesthetic reasons. But for testing purposes at least you could use resistors. It might be a good idea to have an adjustable bias supply for this experiment.