By carseat, I assume you mean an infant carrier carseat. By carrier, I assume you mean a Baby Bjorn or equivalent.
I think the carseat will not be an option at 8 months. If you read the manufacturers booklet you will see that most allow a maximum weight of 20 lbs (or thereabouts) and a maximum height of 26 or 28 inches. My infant carseat (Graco) was max 26 inches and even my DS (who weighed under 7 pounds at birth and was 20 inches) had hit the height limit at his 4 month doctor's appointment. Please check your carseat to see what the requirements are. I know 2 people who have knowingly kept their children in their infant carseats because it was more convenient to do. They believed it to be a pain to have to take their sleeping child out of a regular carseat (they would wake up) or to have to prop them with blankets in a shopping cart (so much more convenient to have the infant carrier). That is not safe IMHO and I make sure to tell my friends who are new moms about the limits just in case they didn't read the entire book and don't know.
DS also outgrew his Snuggly (front) carrier pretty quick. I never tried a carrier on my back and he would have been too big at 8 months for me to think about trying it.
So, as you may have guessed, I voted stroller. If you plan to travel a fair amount or you live in the city you may want to invest in a decent umbrella stroller that reclines for smaller babies, but will also fold up easily on buses. If you are driving, a full size stroller would work too. Honestly, you can buy a decent stroller for not a lot more than the cost to rent one and if you are flying, you will have it for the airport too.