Aha....
The Radian65 is actually one of the least expensive high weight harness seats -- Radian80 is designed for obese kids: the average child will outgrow either at the same time by height (so unless a child is obese, it doesn't seem worth the extra expense & it rather annoys me that just because a child is obese they'd have to pay more)

I do really like the
Radian, but it is much more difficult to install properly in more vehicle types than is
Britax.
Britax convertibles have lower top harness slot height than
Radian, though. The
FF only
Regent keeps kids harnessed the longest, but is not FAA approved for travel (it's just too darn big + heavy to want to use for travel anyway) & they've recently changed the installation recommendation so I can't suggest it just yet unless the vehicle has easy to use reclinable seatbacks (recline the vehicle seatback, install the seat as per instructions, then bring seatback upright again for a nice easy secure fit).
The
Safeguard Go is another option, but it's top harness measurement is more similar to the
Britax convertibles & then only becomes a backless
booster: fine for travel, but dedicated highback boosters that have deep head wings filled with EPS/EPP foam like in bike helmets are safer in side impact crashes (less common than frontals, but far more dangerous).
The Fisher Price Safe Voyage Deluxe is based on the
Britax Marathon without as many frills for slightly less cost & there's also the
new Evenflo Triumph
Advance with similar limits but super easy to use features
The best advice I can give you is to just see if a Babies-R-Us or BuyBuyBaby nearby has most of these options in stock to test out. The most important thing is simply that you can use the seat correctly 100% of the time. Whichever seat you choose based on ease of use for you + kiddo's comfort (for added cooperation) as well as your budget, just make sure a local technician helps make sure it is installed & used correctly.
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 | 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. | |  |
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