Which is better for movies?

The onn 5.1, because its rear satellites and wireless subwoofer produce real surround, and it decodes DTS:X as well as Dolby Atmos. The ULTIMEA relies on a 5.0 layout with a wired sub.

Which is better for a small room?

The ULTIMEA Apollo S60. It weighs 9.26 lbs, mounts on the wall, and its detachable towers tilt 7° so you can aim sound without stands.

Does the ULTIMEA have rear speakers?

No. It is a 5.0 system with five drivers in the bar and a wired subwoofer. The detachable halves are angled towers, not discrete surround channels.

Is the onn worth stepping up to over the ULTIMEA?

Yes if surround and DTS matter to you. The onn adds satellites, a wireless sub and DTS:X. Pick the ULTIMEA instead if wall mounting or app EQ is the priority.

Which has better connectivity?

The ULTIMEA lists Bluetooth 5.3 plus HDMI ARC, optical, USB and AUX. The onn offers Bluetooth, HDMI, optical, USB and AUX without a stated Bluetooth version.

Do either support Dolby Atmos?

Both list Dolby Atmos. The onn also supports DTS:X and DTS Virtual:X, which the ULTIMEA does not list.

ULTIMEA Apollo S60 Soundbar vs Onn 5.1 Ultra-Slim Soundbar System

The onn 5.1 is the more complete surround system: true rear satellite speakers, a wireless subwoofer, and Dolby Atmos with DTS:X support. The ULTIMEA Apollo S60 is a 5.0 bar with a wired subwoofer and a detachable design that splits into two tilted towers, which suits small rooms and wall mounting. Owners rate the onn around 4 out of 5 across hundreds of ratings, praising the value and easy Roku TV setup while noting modest bass and build. The ULTIMEA is heavier per piece and its app control adds tuning the onn lacks.

Choose the ULTIMEA Apollo S60 if you want a wall-mountable bar that splits into two angled towers and you prefer app-based EQ control.
Choose the onn 5.1 if you want real rear speakers and a wireless subwoofer for surround sound and your TV is a Roku TV.

Prices

ULTIMEA Apollo S60 Soundbar: $77.99 at eBay (checked today)