Best convertible car seat??

I have a son who is 9 months old (& is already over 25 pounds) and I am due to have another little boy in December, so I plan on buying my first born a convertible car seat and my youngest will use the snap and go car seat.. But I am at a loss as to which one to get!! Lol I have a jeep Cherokee so not huge but not to small I just want my son to be comfortable while he is still rear facing. Any advice would be much appreciated! Thankyou :)