Help me pick 3 car seats

[CBB]

We currently have 2 Wizards for our 4yo DD who is 41", 35lb. I just checked the manufactural date for one and it says 11/02. I think they expire 6ys from manufactural date (please tell me if I'm wrong). I haven't check the other one yet but I think the most we have is another year since we got it 1year later than the 1st one. DS is 7.5mo and uses Graco safeseat1. He is on the smaller side by wt and average by ht. I think he'll outgrow the safeseat by ht first when he reaches about 1. Looks like I will need to get 2 booster and 1 convertible in the next few mo.

For DD, I'm considering Frontier vs GN. What are the pros and cons? GN seems to be so much cheaper. Is GN harder to install and the seat less comfortable than Frontier? As far as the width goes, which one is narrower? How are the sizes compare to marathon? I will need 2 seats for DD so I'm really loving GN's price. Another question about frontier, can you take to back off like GN? How many years does each of them expire?

For DS, I'm thinking Boulevard vs decathalon vs marathon. Probably more likely the 1st 2 because of the side impact protection since he will still be very little. BTW, what's the difference b/w Boulevard and decathalon? We have an accord and CRV. Does any one know if I can fit a booster and a convertible side by side in either car?

I'm hoping that there will be britax sale the fall. What do you consider a good deal for Frontier, Boulevard/decathalon?

TIA!

[Joolsplus2]

Decathlon has infant padding, it kind of looks like TSIP, but it's not. On the one hand it doesn't have huge metal LATCH bars on the side, that makes it somewhat more likely able to have a booster next to it, but on the other, but harness adjuster is almost impossible for most people, so that's a big thumbs down in my book. Frontier can't be used backless, but it's good for a full 9 years. It is much taller as a highback booster than the Nautilus (boosters are when you want the best side impact protection, don't worry about SIP for convertible/harnessed seats, but it's a bigger deal for boosters). Nautilus is good for 6 years with the back, the backless part is good for 9. Sunshine Kids Radian is as high quality as a Britax, and you might be able to fit a booster next to it a lot easier (it's extremely narrow, and is also a good price for the RN 65 model...the RN 80 is much more expensive and won't last till 80 pounds, so don't bother looking at it).

You had a lot of questions, don't think this covers them all, and I wanted to toss in the info about the Radians :)