The
Radian would be the safest seat more long-term because it has the high weight limit + higher top harness slots,
but it can be difficult to install correctly: what is the vehicle year/make/model

Would your next carseat be installed in the center or outboard?
The
Safeguard Go meets the same safety standards as every other carseat & the higher harness weight limit (compared to the Chase or
CarGo) is more ideal. However the Go is more suited to travel purposes because once the harness is outgrown it becomes merely a backless
booster. Since we do not yet have side impact crash test requirements in the U.S. I don't know how the Go might perform in such real life crashes. I do know that highback boosters are about 70% safer in side impact crashes even without deep head wings, but more dedicated boosters (such as the
Britax Monarch,
Compass B510,
Graco Airbooster,
Recaro Vivo, etc.) that
do have deep head wings filled with energy absorbing EPS/EPP foam offer even more protection.
Harnesses are safest in side impacts, though & it stands to reason that one with a sturdier back would probably be safer than one without, but we don't have comprehensive enough comparative data to know exactly how much safer
Have you considered the Cosco Apex? So long as the vehicle offers head support at least up to eye level (as the child grows) then this would be a good choice

It has the same limits as the Go, but with a firm back + some head wings (it's also lightweight enough for travel)
 |  |  |
 | SafeGuard - Click for more info |  | Indiana Mills and Manufacturing (IMMI) is SafeGuard’s parent company, and has been in the business of developing and manufacturing quality safety systems for every facet of the transportation industry for over 40 years, covering everything from the heavy truck and agricultural industries to child seating.
IMMI has a world-renowned reputation for creating and promoting the highest rated safety harnesses and restraint systems available today, from integrated child seats in production vehicles to campaigning for increased use of safety harnesses on public transportation systems. Based in Westfield, Indiana, SafeGuard has been creating and testing essential components used across the child safety seat industry through their state of the art crash testing facilities since 1961. The SafeGuard and SafeGuard Go are IMMI’s first direct foray into the child safety seat market, and the results are simply outstanding. No other car seats or boosters are designed with as much attention to structural integrity, ease of installation, and total child safety. This seat has been subjected to the most thorough and extensive testing “ever conducted in the industry”: virtual prototyping with computer models; rigorous front, side, and rear impact crash testing; and environmental testing in a range of climates and road types. | |  |
 |  |  |