I agree that the weekday system is preferable. I'll list some pros and cons, but first a note:
The weekday/weekend election is most relevant for 1st year (where you take core classes) and slightly less relevant 2nd/3rd year (where you take electives just based on whatever the elective schedule is). You will find that in most 2nd/3rd years, the weekend people will migrate into some weekday classes, simply because more elective slots are available on the weekday, and thus more classes are available on the weekday.
So, onto the pros and cons:
Weekday Pros: 1st year, your classes are 10 weeks long, so it's a lot more digestible. 1st year, many of the more popular professors teach Weekday, but this isn't overwhelmingly true. Initially, when you get into 2nd year, Weekday people *might* have a slight priority for choosing weekday electives. There are more weekday elective choices available, so it becomes a slight advantage to them, only in popular classes with limited space (for most classes, there isn't a ton of competition to get in). It may be a pro or con, but Weekday students only take classes at the Haas campus (good if you live near Berkeley), where as Weekend students split time between Haas and the south bay satellite (good if you live in the South Bay).
Weekday Cons: You get no spring break. 1st year, you start school earlier than everyone, and end after everyone is done, well into everyone's summer vacation. No free food. You have to commute to campus 2x/week instead of 1x/week (commuting to campus takes up a significant amount of your weekly time, since traffic around Berkeley is horrendous). Weekday cohorts are slightly less social, because people are tired from doing work+class, and aren't quite as eager to get beers afterwards.
Weekend Pros: Very social. Instead of interacting with 1 cohort, you have the opportunity to interact with 2 cohorts every Saturday. Don't underestimate the value of the 'social capital' you get from this... A lot of Weekenders carpool: more opportunity to make friends. 1st year you get the full summer break and spring break. Weekend people tend to be slightly more skewed to the Silicon Valley crowd, so if that's the industry you want to live/breath, Weekend is slightly more skewed in that direction. You're given free food!
Weekend Cons: You're given free food! (yes, it's free, but you do tend to get sick of it) This next point, I can't emphasize this enough - classes are 7 weeks long 1st year. This is *GRUELING*. Core classes are like drinking from the firehose, and in weekend classes, the water comes 30% faster. The classes go 3 weeks, midterm, 3 weeks, final, like clockwork. You tend to take 2 finals on one day at the end, while in weekday classes, the finals are in different days. Despite the easier commute to the south bay campus, a lot of students actually prefer the academic environment of the Haas campus. The Haas facilities are very nice, whereas the south bay facilities feel like a small, stuffy trade conference. The latter part is changing though, since Haas has switched south bay facilities to a more college oriented site.
2nd/3rd Year, everybody goes into electives, so the differences subside. The electives are distributed over what amounts to 6 'slots, M, T, W, Th, Sat Morning, and Sat Evening. Obviously, this means 4 slots for weekday vs. 2 slots for weekend, which means that there is more scheduling and availability on weekdays. So, you generally see more weekend students bleeding into weekday classes, but the reverse happens also. Most elective classes are 10 or 15 weeks long, much more leisurely paced. 1st year is a gauntlet, and the weekenders run it much faster. That also, of course, means that they are drinking, celebrating, and socializing the end of classes when the weekday cohorts are still slugging it through.
