CB would probably be the longer walk, and depends also on where your room will be in the resort. We like to have the walking option when choosing a Universal hotel, but so far, we've had very good service most of the time with the busses. Although I don't think we've ever done the bus from Aventura to get to parks, we always walk, or catch the water taxi at SF. But bus to/from CB was great.
As tempting as it would be, I wouldn't cross the road from CB to Aventura. Am sure many have successfully done it, no issues, but as someone driving that road, the crossing is just after a bend in the road, and since no crosswalk, drivers might not at all be considering people walking.
The hotels have 2 different vibes for sure. CB reminds me more of like a value/moderate at Disney, and I found it full of kids/families, etc. which is fine, but a lot noisier. But lots of fun stuff to do there, and DD enjoyed the pools/lazy river. We lucked out too w/ a good room location, but I could see how some might not be ideal. We love Aventura and have stayed several times. Rooms are basic/no frills at all, and not really family oriented, but we've enjoyed our stays, food and pool were fine, and it's so close to SF, can very easily get there.
Both are good options, just different.