Welcome to the boards.
My own daughter moved to a Husky (same as Regent) at 38 inches tall and 35 pounds and did just fine in it. She slept comfortably (meaning, she didn't complain, but her head
did slump a bit). However, if there had been a seat out with top slots the height of the Radian at that time, I would have chosen it over the Regent simply because it's smaller all the way around, while still fitting most kids to age 6-8. I can't comment on how well kids sleep in the Radian because mine never has.
At 30 pounds she could still rear-face in some seats, including the Radian. That might be something to consider, as rear-facing is safer than forward-facing. It's tall enough that even long-torsoed kids should be able to rear-face to the 35 pound RF weight limit. Rear-facing is definitely the most comfortable way for car sleepers and young kids to ride because they are reclined (no head slumping).
If she will stay forward-facing, other seats to consider (aside from the Regent and Radian) are the Graco Nautilus (brand-new, in some Wal-Marts now, should be more widely available in the coming weeks), Safeguard Child Seat and Cosco Apex.
What seat is she in right now? Where are her shoulders in relation to the top slots?
You could also consider a convertible seat for her (like the Radian) even if she will be forward-facing, with the thinking that when she outgrows it in 2-4 years and moves up to a booster the new baby could move into the new seat. Great ones are the Evenflo Triumph Advance and Britax Marathon.
My own preference would probably be a Radian, and rear-facing. If forward-facing, I'd choose the Graco Nautilus most likely, unless I was sure I wanted to harness beyond age 6. If that's the case, go Regent.
The Britax convertibles tend to not fit many newborns well because the shoulders are just too far below the lowest slots. Other convertibles have lower bottom slots than Britax (Radian, Evenflo Triumph Advance, Cosco Scenera, Safety 1st Uptown are all good choices for newborns). If you tend to have bigger/longer-torsoed babies a Britax convertible may work, but if slots are TOO far above shoulders (over an inch or so) and/or the harness won't tighten down enough, a convertible won't work from birth.