How important is fresh ground coffee vs a good coffee grinder?
Both have downsides:


Coffee that is ground more than a few hours before brewing loses aroma, which is obviously an important part of flavour. 
A blade grinder doesn't produce evenly-sized particles; big particles will under-extract (losing flavour) and small particles will over-extract (introducing bitterness).


My wife and friends didn't believe me about the over-extraction potential of blade ground coffee, so we did a blind taste test between coffee beans ground with a blade and ground with a burr. I identified the blade-ground coffee every time. That's not the comparison you have, but in the absence of a burr grinder I'd go for pre-ground, for convenience.