We have used both the SD40 and an SD90. We prefer the SD90. The overall sound? Pretty much the same. Yes, the 90 does offer some additional sounds and Live guitars., but most sounds are identical.
The 90 offers a sampler – Works well if you have a source for samples. We did (still) provide some vintage organ samples. The 90 also offers the Launchpad feature, which you don’t hear about much or see folks using. We messed with it and considered releasing content for it but decided against that when looking at the time invested vs. $$$ returned on the effort.
The primary reason we prefer the 90? The front panel and access to controls. We find it easier to operate using the sliders vs. the smaller rotary dials on the 40. There are also a few additional physical controls/buttons on the 90, where on the 40, that same function is accessed from menu “X” or worse, SUB-menu “Y.”
What is all of the above “worth”? Ask five people, and you will most likely get eight different opinions!
If you are interested in a “previously enjoyed SD90”, drop me a note (firstname.lastname@example.org). Maybe we can work out a price that works for both of us.