Choosing a class just because anyone tells you that its currently needed, makes no sense at all. Joining a guild because its currently recruiting that class makes no sense at all. Environments change monthly, that goes for most guilds.
What a raiding guild needs is people with strong personalities, ability to team-work, consistency, a will to play long-term, activity, dedication and the list goes on. Play the class you want to play and if you have the above mentionned things, you will be more valuable to most guilds than 90% of the players on the server.
About what guild to choose.. Read their recruitment messages, talk to people, check raidstats, their websites and so on. Chose the guild that has a suitable image and give your best to apply. The lower the recruitment status, the better you have to be to raid right away, if the recruitment status is closed, you might aswell try a social spot and show a little dedication and wait for a month.
I know this doesnt answer the authors question, but imo it should have never been asked that way