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The only preparation that I can really envision for U46 is if you want to fairly quickly get into subclassing I should think you'd want your base class skills and all likely support skills already maxed and plenty of extra skill points banked up. To me that suggests not creating a new alt since it will take a new alt much longer to get fully ready.
If you have already decided you want a new alt, then the choice depends on why you want one.
If it's variety, then multi-class characters are going to be a new thing, and guarantee that. If you want to try another skill line, then the choices are an alt now, using the class that has it, or wait for 46 and mix it with ones you know. An alt of a class you haven't used before throws three skill lines at you at once, which may also be a factor.
Combining these with the post above, if you want a multi-class new alt, then do it now, as you can at least build up the lines you'd keep from the base class, and not have to do it all when 46 drops.