1) Totally depends on how complex things are going to get. If you want a quick DAL that more or less mirrors your tables in a 1:1 fashion, go for L2S (or SubSonic if you want something more mature and supported). If you are going for more of an n-tier type thing where your tables and domain model are completely different, go for an OR/M like Entity Framework (or NHibernate if you want something that is pretty much better in every way)
2) ASP.net Membership is extremely complex, and there are bits of it that are fairly poorly engineered. However, it depends on how much experience you have with these things. If you enough to know how to take steps to avoid session fixation attacks, just roll your own because chances are it will be better then the canned solution. If you have no idea what that is, take the time to learn the default one.