If you have to choose, go for the scope first. The LA will come in very handy for logic problems on boards, but it's useless for dealing with power supply issues. Honestly a lot of logic issues can be diagnosed quite well with just a scope or logic probe. You only need the LA when you have to see all the data on a bus or something - if you just need to know all the lines are moving, the scope or logic probe will give you that info.
That said, if it's cheap enough I sure wouldn't be able to turn it down.