Block Party Bash actually has two "show" areas in the same parade, and stops at each one for 10 - 15 minutes. The first stop is the Sunshine Plaza (at 5:15), and the second stop is over by the Golden Dreams theater (around 5:30). When we were there in early May (a slow weekend), Sunshine Plaza was already packed with people staking out spots when we arrived at 4:00, but even at 5:00 NO ONE was waiting over by Golden Dreams. Aladdin is around 40 minutes, so you could see them both in one of two ways (both will be a squeeze...)
1) Go to the 4:05 show of Aladdin, then afterwards dash over to the Golden Dreams area for Block Party Bash. The only problem here is that you have to go through Sunshine Plaze to get to Golden Dreams, and it will be packed with people waiting for Bash
2) Go to the 5:15 Bash at the Sunshine Plaza, and stake out a spot early towards the Hollywood Pictures Backlot side, and then when Bash is over RUN to the 6:00 show of Aladdin.
Personally, I'd go for option #1, because Aladdin is more of a must-do for me AND you can still enjoy Block Party Bash, even if you get there a little late.