A few comments that may be useful:
The RapidTurn has a spindle sensor so threading is supported. There is no spindle sensor so no threading if you are vertical turning.
The RapidTurn comes was a 5C mounted 3-jaw chuck and you can also use a 5C pot chuck. The mill spindle is R8. I've never seen an R8 mounted 3-jaw but someone probably makes one.
The RapidTurn uses 5C collets which can pass a 1-inch rod which may reduce waste when making your smaller washers. With vertical turning you will waste an inch or so of each piece of stock since the drawbar prevents passing stock up the mill spindle..
With the horizontal axis of the RapidTurn you may get better chip removal (gravity assist) when parting. With vertical turning you'll probably need an air blast to clear chips.
You didn't mention material but with steel both the RapidTurn and the vertical spindle may have insufficient low speed torque to take a reasonable DoC.
Watch Keen's videos to get some ideas of what is possible with each choice. If washers are your only product the SlantTurn lathe is probably a far better choice.
Just a few quick thoughts. I'm sure that Keen and others will chime in.