I have been members at both.
The Y has great programs, alert life guards, excellent instructors, all in a community based environment. The Y is safer in the pool area- things are less slippery and there are large rails and steps that lead into the pool. There have been 2 on-duty life guards during all the morning weekday water aerobics classes I have attended at the Y.
Allstar is a nicer facility. The staff at Allstar is generally not friendly. Allstar has an emerald city smoothie, while the Y has modest snacks and coffee.
We had a bad experience at Allstar: the salesman sold us on a month to month contract. When I went to fill out the paperwork, he said not to worry about certain sections... this happens sometimes so I didn't worry about it. Big Mistake! I made a huge error by not insisting he finish his areas before signing because he filled the paperwork as a a year contract. We decided after a couple months we wanted to end our membership and put in notice of cancellation and got resistance. We tried to resolve this with management but it took over 3 months. Management didn't return calls, they were rude, unprofessional, and only responded when I mentioned that I would be contacting "get Jesse." The accountant was extremely rude as well. We ended up being forced to pay for over 3 months of additional time- the amount of time it took to cancel the membership. The salesman had been fired before we even started to try to cancel our membership so he is no longer there. My advise to you if you you go with Allstar is to be careful with the paperwork!
My vote goes for the Y! It has that strong West Seattle community feel, is safer, and has excellent classes.
Good luck with your selection and I hope this was helpful!