With 5.5 PH/s I would go solo... At the moment, this would produce about 3.5 BTC/week. Always depends on the costs of power, of course.
VERY bad idea. At 5.5PH your solo variance would be astronomical.
You firstly currently would average more than 60 days before you find a block (though the next diff change will be over 10% so it will be longer and include many more diff changes).
And if you hit over a 200% block (which is expected 1 in 7.4 blocks) that'll be more than a 1/3 of a year away, with no income until you find a block.
5.5PH is not big time - it's quite small compared to the bitcoin network which is currently over 47976PH.