Based on our experience last year, I would definitely do the afternoon thing.
We went to BB in the morning, well before rope drop. It was much more time consuming b/c we had to stand at the turnstile, then stand at the rope drop, etc. And of course the lines build quickly. The line for the lift to Summitt Plummet got long very quickly (we were there very late Aug/early Sept). If you do go at opening, don't set up camp anywhere near the front of the park. Everyone is in such a hurry to get onto slides that the seating gets crowded anywhere near the front half of the park. We were packed in like sardines but when we went on the lazy river, we passed lots of more open spots with plenty of chairs towards the back. ALSO, if at all possible, USE THE SINGLE RIDER LINE for the lift! My teens were able to do it over and over b/c that enabled them to walk right to the front of the line. Its only a two minute ride so who care if you have to sit by strangers for a few moments. Its worth it.
We went to TL in the afternoon and that was much better. Again, take your time finding spot far back. It was packed all around the big wave pool, but lots of open spots around the shark viewing.
FYI, we thought that in general the slides at BB were better than TL. The thing they like the most at TL was catching the big waves (but my teens are competitive swimmers...other than them and a few 20-something guys, most people had a hard time with the endurance needed to do it and just ended up being crashed on by the waves.)