The Wega has a 2 liter boiler according to the spec sheet while the Oscar lists a 2.3 liter boiler so once these are heated up they should be close. The Wega has the E-61 grouphead (not the entire group) so it is similar to your Bunn ES-1A in this area with more mass than the Oscar's group. The Wega has a hot water feature (easier to drain the boiler from what I have read) for tea, Americano's, etc. Wega has a boiler pressure gauge while the Oscar does not. Metal case on Wega, plastic shell on the Oscar.
From my research these are the main differences in these HX machines. I suspect the group, additional gauge, and metal body, and smaller manufacturing capacity contribute the most to cost differences but I am not an expert - I am fortunate to have some time to research this choice so I want it to be the best one for me.