Hello again
When DD outgrew her infant carrier at 7 months we looked at the options and, at the time, could only find the two you mention. We saw and fitted both of them in the car and the Britax was just too large for our car (Audi A3 Sportback).
We were pretty happy with the Ultimax, it fitted well and the sealtbelt routing meant it was possible to get DD in and out (something that was nigh on impossible with the Britax), but DD wasn't happy on long journeys by the time whe was about 15 months (we drove to Burgundy so about 8 hours in the car. Once we got there it took us over a week to be able to put her back in without tears and tantrums ) She was also starting to look big in it. By the time she was 19 months we'd replaced it with the BeSafe Izi Kid, and then promptly drove her to Scotland (also about 8 hours) - and this time she was absolutely fine the next day. I dont' know whether its because its higher (so she can see out the window), because she's got more leg room or whether its just a more comfortable seat for her.
If I were using it for a newborn, I'd go for the Concorde - as NorktasticNinja says the BeSafe is pretty upright. But don't be suprised if your DC becomes unhappy in it RF before they outgrow it in that position.
wrt the sealtbelt length, the BeSafe will require much less sealtbelt as it doesn't route around the back of the seat - which happens for both the Ultimax and the Britax. The BeSafe feels much more secure than the Ultimax did.
Have you come across the contact details for the Essex County Council InCar Road Safety Team (here)? They were very helful when we were trying to decide what to buy (even though we're in Hampshire) and have no vested interest in trying to do anything other than keep your DC safe.
Good luck