Best FF seat to remain harnessed?

I need to figure out a new seat for my 3.5 year old daughter. She is 42lbs and 41 inches tall and her weight is increasing too quickly to make the Radians worth it simply to RF for another few pounds. I need to keep her harnessed for as long as possible since she has a mood disorder and cannot sit still on her own, my goal is to keep her in a harness until she is 7 years old if not longer. She was in the Graco Nautilus I purchased last year in order to move her Britax Marathon into my Dad's truck but I want something built better, the fabric has already worn through and it's less than a year old, besides it only goes to 65lbs as a harnessed seat.
Im debating between the Britax Regent, Sunshine Kids Radian 80, Sunshine Kids Radian XTSL and the Britax Frontier 85.
Which has the highest harness slots?
What is the SuperLatch on the Radian?
I drive a 2006 Dodge Grand Caravan and can place the seat in a captains chair or on the bench in the rear, wherever it fits best.
I don't know enough about any of these seats to make a decision at this point. What are the pros and cons of each? Are there any which may not install well in my van?

I'd look at the Frontier 85 when it comes out. It has 20" top slots, 2.5" higher than the Radians (which are often outgrown around 60 pounds). It also turns into a booster, which none of the others do.

The Regent has top slots about the same, and an 80 pound harness limit. No booster. But the price is nicer than the others.

If you're going to forward face I'd ignore the Radians.

The SuperLATCH means you can use LATCH to the limits of the seats, rather than the 48 pound limit your van has, if your vehicle is made after September 1, 2005 (for your vehicle you'd have to check when it was actually made. It may be before or after that date).

Wendy

Thank you, Graco has a new cover for the Nautilus in the mail now. Im putting that one in my Dad's truck so it gets less use, I'll use the Marathon until she outgrows it then go for the Frontier 85. I checked my van and it looks like the SuperLatch would be worthless for us anyways. Thanks for your help! Next time I'll go straight from the Marathon to the Frontier 85