Need a new seat ... almost 5yo into a booster?

Ok .. So if I saw this title I'd be thinking "NO...do NOT booster your "4" year old." (He will be 5 in Oct) In fact, I keep telling myself this. But this guy has nearly outgrown the Diono100 by height and the only other thing he fits in harness-wise is the Frontier which he A) HATES and B) doesn't fit well in the MDX.

My Radian65 died and it's going to need to be replaced by either another harness seat or a good booster. I technically only have one other kiddo that needs harnessed ... Miss A is 2 1/2, so she will be harnessed for awhile, but other than her, I only have incoming foster kiddos to harness. Currently a 5yo who is in the Frontier in the Land Yacht and a Maestro in the MDX.

So here's the seats:

Maestro(2, but the other is in dh's truck)
Frontier
Diono100
FPSVD (Expires in March of '13)


B510
Monterey
ProBooster
FPSV Booster
Backless Boosters

I've pretty much decided I'm going to get a new seat, or two. I'm just not sure if I should put out another $200 to get a Diono100 that will only last L another 6mos (he's almost outgrown it), or find a booster he fits well in and go with that. He has ridden in them some and does a very good job sitting in them. The foster 5yo does not, which is why he's in a harness and not being considered for boostering(plus he's smaller than L). Also, to clarify ... there are two vehicles we use and I don't mind moving boosters, but it drives me batty to have to move the car seats back and forth ... so I guess I'm trying to decide if I should buy another harness seat to install and leave, or go ahead and get a good booster that he fits well in. And in the MDX, there's not a ton of room, thus the Diono100 (plus I love them ) ... so I'm not sure what booster I'd get.

AND ... I still feel like age-wise he seems a bit young for full-time booster riding, but the stinker has such a tall torso, I'm not sure purchasing another harnessed seat makes sense.

BAH! Help me out!!!

He can continue to ride in the Diono until the tops of his ears are level with the top of the seat even if his shoulders are above the top harness slots.

I would not booster him if I owned harnessed seats that he still fit in or had the means to purchase another seat. I couldn't imagine my 4 year old going into a booster in a few months. It would be a disaster!

The new Evenflo convertible has 19" top slots, is $100 and should be in stock at Target soon. That is another option over a Frontier.

Which seat is that? The other issue is the space in the mdx, and that he pretty much needs to be in the center because he gets car sick so easily I love radians/Dionos, I'm just not sure I can convince hubby that I need another $200 seat for 6ish mos of use. My girls used the radian 65s til they were 6 1/2! He's definitely not going to last that long, even in the 100 .

I can always continue to move his 100 back and forth between vehicles, I just didn't want to anymore!