Strictly preference; I prefer DSA on an ergodox. The flat plane I think is a feature.
Use whatever you like; nobody here is going to force you to set your keyboard up any particular way.
But seriously, having the further-away keys be taller is a real functional advantage, and on a column-staggered layout in particular makes a pretty noticeable difference. I’ve polled maybe a dozen people who tried DCS and DSA keycaps on Ergodoxes back-to-back, and nearly all of them preferred the DCS version (the others were indifferent and didn’t like the Ergodox at all). [Admittedly, this was a short test, only maybe 5–10 minutes typing on each version. Also, these weren’t keyboard “enthusiasts,” just a random group of people in a coffeeshop one day. Definitely not the most rigorously scientific study ever.] If you haven’t tried a set of sculpted keycaps on an Ergodox or similar keyboard, I strongly recommend trying it for a day or two before dismissing the idea.
Beyond that, the Ergodox has IMO somewhat poorly designed thumb sections (at least for small to medium sized hands), and 2–3 of the keys in each thumb section are basically unusable without extra-tall keycaps on them.
DSA does have one big advantage, of course, which is that you interchange the keycaps between rows, which is very convenient if you have a non-QWERTY layout you need legends for, or want to get the right legends on all the modifier keys, etc. Also, some of the DSA keycap sets are very nice looking.
* * *
Edit: Reading it again, I think this post sounds more negative than I intended. I have strong opinions about which is better in the abstract, but I really don’t have any problem with people using DSA keycaps if they want to. If something works for you, that’s great. Someone getting a new Ergodox might want to try both types of keycaps for themselves.