You cannot get AA experience until level 51 so get level 51 first before making a decision. It also depends on your play style. Personally, AAs become more important after level 60. I think this has to do with the fact that AAs were not even invented until the level cap was 60 and many players had no more advancement options. The 50s are long enough, I suggest going for 100% xp until 60. Then try 90%xp 10%AA or 80/20 or 50/50 or 0/100 if you really want them. At 65, start seriously considering getting 2 dozen AAs or so to fit your needs.
As for groups, the Hot Zones like the Hols and the Planes of Power will be your new home.
Soloing is trickier in that it all depends on what you can handle and in what type of environment. You have to consider the possibility of adds, NPC resists, whether or not you can kite, if you can root/rot, if they summon, etc.
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After 16 years, I'm not listing every friggin character.