I have decided. I've worked out a way to modify my design a little to use my existing stepper motors with the dual extruders for now and allow for a geared motor and dual drive gear in the future. thanks all

Thanks Guys, you all have given me more to think about. The geared 17HS19-1684S-PG5 stepper motor is only US$30, so cost is not an issue. (The extra weight is about 200g each motor) I have a large bed 500x500. and may need to allow to upgrade to a dual drive geared extruder feed in future.
If I don't change my design to the geared system now, I will not have the space available be able to upgrade.

I am finalizing constructing a Duet2 system.
I have a dual extruder to setup and have a choice of either using 2 x 17HM19-2004S (0.9deg) direct drive that will give me 188 e-steps per mm at 16 micro-steps or 2 x 17HS19-1684S-PG5 (1.8deg) geared drive that will give me 488 e-steps per mm at 16 micro-steps.
Notting that the geared drives will add a little extra weight to my carriage and I have make a 20mm coupling for each of the geared drives, and modify the design my carriage.
Is it necessary to go for the greater steps? or put it another way - Would I have a better printer with 488 e-steps per mm for my extruders or is 188 good enough for 1.75mm filament.