Speed ratings are almost completely meaningless. I would view those two tires as identical.
As for width, you don't really want to raise the external diameter of your tire much. Also, your rim is only 6.5" wide. 215/55/16 is .4" taller and a whopping 30% wider than your rim. Especially with squishy all seasons like you suggest, you really don't want to go much over 25% wider tire than rim. Your sidewalls would be like /
Personally, I'd save some bucks to get a wider rim and a better tire.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD