Re: Help! which one to buy for older harnessed child?
Just FYI, he has likely outgrown the chase already. Besides the 40# weight limit, the child's shoulders need to be under the top harness setting. I think that the vast majority of 42" children will have outgrown it several inches ago.
If you want to keep him harnessed as long as possible, then the Britax Frontier is really the best choice. It is usually on sale somewhere for around $200.
Do keep in mind, with whatever you choose, that seats are typically outgrown by height long before weight. The kids shoulders cannot go above the harness (when forward facing), so comparing the amount of space between your son's shoulders and the top harness setting will give you an idea of how long various seats will work, and which ones will last longest. (The frontier has the tallest harness of any seat on the market.)