Both are stiff. Both are brighter than most other flats (at least out of the box). None of them are particularly thumpy, even after they mellow out.
That's where the similarities end.
Roto flats have a much rougher surface. They are sort of between other flats and roundwounds. D'Addario Chromes have a much smoother surface, like most flats.
Roto flats have a very strong mid presence, and are quite bright when new. Not "zingy" bright, but "upper mids" bright. The brightness of Chromes is a much "narrower", clanky/metallic sort of brightness. Basically "the D'Addario tone". Compared to the Rotos the brightness of Chromes is more about string noise/clank than an actual tonal upper mid presence. Both will mellow out over time.
Rotos can be set up with lower action than most other strings without introducing fretbuzz. The heavier gauges they also NEED to be set up with a lower action than most strings. If not, they will be very hard on your fretting hand. D'Addario Chromes work best with a "standard" setup, like most other strings of similar gauges.
Roto flats have a noticeably lower output than other strings (due to the monel wrapping). The output is even lower than pure nickel strings. D'Addario Chromes have a normal output, maybe even slightly higher than average.
Out of the box Rotos sound almost like roundwounds. They mellow out over time, and will eventually sound closer to other flats, but the strings typically mellow out very differently. The E+A-strings stay bright for a long time, while the D+G-strings mellow out relatively fast. A fresh set of D'Addario Chromes sound like flats with added "clank" and mellow out more evenly across the strings.
Ok, that's the objective differences. Now for my highly subjective opinions:
Personally I love Roto flats. The mids of Roto flats are simply gorgeous! But I think of Roto flats more as an alternative to roundwounds, than an alternative to other flats. They're totally unique.
Personally I hate D'Addario Chromes. I find them extremely inarticulate. I also find them too clanky when they're new and dull sounding when they mellow out. EXCEPT the short scale sets, where the stiffness is counteracted by the lower scale/tension. Chromes are much "livelier" on short scale basses. I actually like Chromes on short scale basses. They're not my favorites, but I like them.
With Rotos it's the opposite. I don't like the short scale Roto flats at all. In fact, I find them utterly useless. Far too floppy and buzz like crazy with anything close to normal action (at least with MY playing style).
So basically, I love Rotos and hate Chromes on long scale basses, but like Chromes and hate Rotos on short scale basses.
I can't predict which ones YOU will prefer though. These strings are so TOTALLY different that you will 100% definitely prefer one over the other.