Hi, great question. Let me add my two pence worth as someone who had a X20 and now has a X30. Overall, both are great cameras if the user wants a view finder, some control over exposure and a smallish, light body with great image quality (for the sensor size).
As far as I know both have the same lens and sensor so apart from a few JPEG tweaks the image quality will be the same.
The EVF, in my opinion, is far better than the OVF. As a long standing OVF user I was sceptical about EVFs, but the current generation (I also have a XT10) is now of sufficient capability to be very useful. Not least is the ability to see the effect of any exposure and film simulation adjustment in real time before one takes the picture. Furthermore, I always use a lens hood and the one designed for the X20/X30 intrudes into the optical field of view a little; no such issue with the EVF.
The body of the X30 is marginally bigger on paper than that of the X20, but in practice I find a negligible difference. I found the battery duration of the X20 insufficient for a medium shooting day so always used two batteries. With X30 I have found the battery able to cover a medium shooting day (but I always carry a spare).
For me, given the same image quality, I would balance the advantages of EVF and battery versus cost. I am very pleased with the X30, which I use mainly for hiking and family events. Interestingly, I have only recently bought mine (in August). I could not find a new one in a UK seller, but there are some new ones in Japan at very high prices as the stock dwindles. I was very lucky to find a mint used one. They are obviously still popular cameras, which does not surprise me.
Hope that helps.
